- There is one and only one God.
- God eternally exists in three distinct persons.
- The Father is God, the Son is God, and the Holy Spirit is God.
- The Father is not the Son, the Son is not the Father, the Father is not the Spirit, etc.
(Image source: Justin Taylor)
- Visual Theology - The Trinity - low resolution, high resolution(image), by Tim Challies
- Fred Sanders
- The Trinity of Persons: Unity and Distinction, Equality and Differentiation, by Bruce Ware
- What is the doctrine of the Trinity?(MP3), by John Piper
- How can Christianity be both monotheistic and Trinitarian?(MP3), by Tim Keller
- "This is My Beloved Son": Exulting in the Trinitarian Relationships of Jesus Christ (MP3s) - Bethlehem Conference for Pastors 2005
- Equal in Essence, Distinct in Roles: Eternal Functional Authority and Submission among the Essentially Equal Divine Persons of the Godhead (MP3), by Bruce Ware.
- Jesus and the Father: Love and Obedience, Authority and Submission, by Bruce Ware ( PDF)
- Jesus and the Spirit: Trust and Reliance, Exaltation and Glorification, by Bruce Ware ( PDF)
- Conference Speakers Q & A, with Ken Jones, Vishal Mangalwadi, John Piper, and Bruce Ware - Includes discussion on various aspects of Trinity
- Lectures by Mike Reeves (MP3)
A brief history
The doctrine of the Trinity was formally developed in the early church in reaction to errant teaching on the nature of God as found in Arianism. Arianism attempted to protect monotheism (the belief in one God) by denying the full deity of Jesus, a belief most Christians held at this time. Arianism taught that Jesus was divine, but that he was a lesser deity than the Father. To affirm the Church's stance on the nature of God, the Trinity was formally stated in the Nicene Creed(325 A.D.) and the later Athanasian Creed. As a result of these early ecumenical creeds, any departure from the Christian doctrineof the Trinity was considered heresy. These creeds affirm the early Christian conviction that Jesus was God. Arianism caused the church to dogmatically affirm what was already believed and inherent to the earliest of Christian theology.
The term "Trinity", is not found in the Bible. Theophilus of Antioch around 180 A.D. first used the Greek term trias(a set of three) in reference to God, his Word, and his Wisdom. However, Tertullianin 215 A.D. was the first one to state this doctrine using the Latin term, Trinitas(Trinity), referring to the Father, Son, and Holy Spirit (W. Fulton in the Encyclopedia of Religion and Ethics).
See main page: Development of Trinitarian theology
Understanding the terms
In Trinitarianism, God is said to be three persons in one essence, nature, or being. These last three terms are often seen as synonymous, but it can be helpful to understand what each means independently. Theological explanations of the Trinity, especially as developed in the early church, included the use of certain Greek terms which are included below for reference.
The word "essence" may be defined as the intrinsic or indispensable properties that serve to characterize or identify something.  Essence describes what it is to be God. The three persons of the Godhead share the same essence, and God's essence is immaterial. With this understanding, the doctrine of the Trinity continues to assert monotheism, an essential and easily found belief within Scripture.
See main page on essence
The word "nature" may be defined as the essential characteristics and qualities of a person or thing.  Sometimes "nature" is synonymous with "essence." Ontologically, each of the three members of the Trinity possess the same essential nature.
See main page on nature
The word "being" may be defined as the state or quality of having existence.  The triune God of Scripture eternally exists and has eternal being. Again, along with a monotheistic understanding, there is one and only one being, that is, God.
See main page on being
The word "person" may be defined as the composite of characteristics that make up an individual personality.  Scripture presents separate individualities of the Father, Son, and Holy Spirit, as distinguished from the "essence" of the Godhead that unites them.
See main page on person
Homoousiosis a Greekterm that means "of the same substance". It was used against Arianismto define the relationship of Jesusand God the Father. They were of the same substance, or in other words, were of the same being.
See main page on Homoousios
The Greek term heteroousioswas used by Arius to describe the nature of Jesus. It means "of a different substance" and thus Arianism seeks to describe Jesus as being separate from God the Father. Reactions escalated and resulted in the Council of Nicea where it was affirmed that Jesus and the Father were of the same substance (homoousios).
See main page on Heteroousios
Used by Eusebius of Caesarea, homoiousiosmeans "of a similar substance". This is in contrast to the Nicene affirmation that Jesus and God the Father are homoousios, "of the same substance." Christians at that time believed that even if they were of similar substance, the result was a Jesus who was not identical with the redemptive God of the Old Testament. Furthermore, if he had a similar divine substance, an immediate problem arises with the doctrine of monotheism. Thus, at the Council of Nicea the church affirmed that Jesus and the Father were of the same substance.
See main page on homoiousios
Perichoresisis a Greekterm used to describe the triune relationship between each personof the Godhead. It can be defined as co-indwelling, co-inhering, and mutual interpenetration. Charles Hodge explains that this term was used "to express the Scriptural facts that the Son is in the Father, and the Father in the Son; that where the Father is, there the Son and Spirit are; that what the one does the others do (the Father creates, the Son creates, the Spirit creates), or, as our Lord expresses it, '[whatever the Father does, that the Son does likewise]' (John 5:19). So also what the one knows, the others know. '[For the Spirit searches everything, even the depths of God. For who knows a person’s thoughts except the spirit of that person, which is in him? So also no one comprehends the thoughts of God except the Spirit of God]' (1 Cor. 2:10, 11)." 
See main page on perichoresis
Essential to the Trinity is that there is one and only one God. It is essential because it was the conviction of monotheism- that there is one God - that drove the early Christians to formulate the doctrine of the Trinity from Scripture. More importantly, monotheism is the teaching found in the Bible.
Scripture is clear that there is only one God: 'There is no other God besides me, a righteous God and a Savior; there is none besides me. Turn to me and be saved, all the ends of the earth! For I am God, and there is no other (Isaiah 45:21-22; see also 44:6-8; Exodus 15:11; Deuteronomy 4:35; 6:4-5; 32:39; 1 Samuel 2:2; 1 Kings 8:60).
Fundamental to the Judaism of the OT (and of today) is the shema. It is found in Deuteronomy 6 and part of it says, "Hear, O Israel: The LORD our God, the LORD is one," (Deut 6:4). The understanding of monotheism is at the heart of this passage, and it was at the core of the early Christians understanding of the nature of God.
Each person of the Trinity is God
The doctrine of the Trinity must remain grounded in God's Word. Roger Olson sums it up when he says,
"While it is true that no passage of Scripture spells out the doctrine of the Trinity, it is also true that the whole of Scripture's witness to who God is and who Jesus Christ and the Holy Spirit are makes no sense at all without the model of the Trinity and that all alternative concepts end up doing violence to some essential aspect of revelation, Christian experience and possibly even reason itself," ( The Mosaic of Christian Belief, p. 139).
The Father is God
See main page: God the Father
Jesus/the Son is God
See main page: Deity of Jesus
The Holy Spirit is God
See main page: Deity of the Holy Spirit
Passages equating the Father, the Son, and the Holy Spirit
Matthew 28:19 - "Go therefore and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit"
- This passage suggests that these three Persons share the same name.
2 Corinthians 13:14 - "The grace of the Lord Jesus Christ, the love of God, and the fellowship of the Holy Spirit be with you all. Amen."
- Here, again, the Father, the Son, and the Holy Spirit are equated. Observe that frequently the Father is spoken of as "God" in reference to the Son because of the covenant between the Father and the Son.
Revelation 1:4-5 - "John to the seven churches which are in Asia: Grace to you, and peace from Him who is and who was and who is to come, and from the seven spirits who are before the throne, and from Jesus Christ the faithful witness, the first-born of the dead, and the ruler of kings on earth."
- The Apostolic Greeting only involves members of the Godhead. The seven spirits who are before the throne is evidently a way of referring to the Holy Spirit who is present wherever the church is present.
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Misconceptions about the Trinity
Colin Gunton(1941 - 2003) asserts that, "Trinitarian theology is not a theory; it is an account of God's being which is tied to his action, and that action centres on a gospel rooted in the life, suffering and resurrection of Jesus," ( Father, Son & Holy Spirit, xiv).
This is a section stub. Please edit it to add information.
Various heresies arose throughout the history of the church, many of which would require significant replies in order to defend the orthodox and biblical view of the one triune God. It is also important to remember that "heretics did not work outside the Christian community - they counted themselves as faithful Christians attempting to explain the gospel in terms their contemporaries might understand" (Gonzalez, A History of Christian Thought, p. 121). That is, some heretics would ascribe to "one God" but would mean something different than what orthodox Christians taught. Thus it was important to work through doctrines carefully and to protect each detail, whether it be monotheism or the humanity of Jesus.
Denial of the oneness and uniqueness of God
These result from a denial of the statement, "There is only one God."
Denial of the eternal deity of each Person
These result from a denial of the statement, "Each Person is fully God".
Denial of the three Personalities or Persons
These result from a denial of the statement, "The Father, Son, and Holy Spirit are distinct, eternal Persons".
Denial of the two, distinct, unmingled natures of Christ
"To illustrate the significance of the Trinity of our faith, consider just briefly the relation of the doctrine of the Trinity to the Christian understanding of salvation. In order for us sinners to be saved, one must see God at one and the same time as the one judging our sin (the Father), the one making payment of infinite value for our sin (the divine Son), and the one empowering and directing the incarnate—human—Son so that he lives and obeys the Father, going to the cross as a substitute for us (the Holy Spirit). The Christian God, to be savior, must then be Father, Son, and Holy Spirit. That is, our salvation comes as the Father judges our sin in his Son, who became incarnate and lived his life in the power of the Spirit as the perfect and sinless God-man, and accomplished his perfect obedience to the Father through the power of the Spirit. Disregard the Trinity and you necessarily undermine salvation."
- Fred Sanders, The Deep Things of God: How the Trinity Changes Everything. Crossway, 2010,
- Robert Letham, The Holy Trinity: In Scripture, History, Theology, and Worship. P&R, 2004.
- Olson and Hall, The Trinity (Guides to Theology) . Eerdmans, 2002.
- Köstenberger and Swain, Father, Son and Holy Spirit: The Trinity and John's Gospel . IVP, 2008.
- Tarmo Toom, Classical Trinitarian Theology . T&T Clark, 2007.
- What Books Should I Read on the Trinity? A Top Ten List, by Fred Sanders, professor of theology at the Torry Honors Institute, Biola University.
- Forsaken: The Trinity and the Gospel, by Thomas McCall - Catalyst 38.3 (2012)
- Doctrine of the Trinity Reading Room: extensive collection of on-line resources on the Trinity (Tyndale Seminary)
- God Is Love Requires More Than One Person in the Godhead
- The Doctrine of the Trinity: No Christianity Without It
- A Brief Definition of the Trinity, by James White
- The Modern Doctrine of the Trinity, by Fred Sanders
- Understanding The Trinity, by Matt Perman
- What is the doctrine of the Trinity?, Desiring God Staff
Historical figures on the Trinity
- An Unpublished Essay on the Trinity, by Jonathan Edwards
- The Unity of the Divine Essence in Three Persons Taught, in Scripture, From the Foundation of the World, by John Calvin, Institutes1:13.
- Trinity, Fountain of Salvation, by Fred Sanders
- Monotheism and the Trinity, by Aaron Goerner
- Trinitarian Analogies, by John Frame
- Think Bigger - There’s not a ‘Trinity verse’ — and that’s a good thing, by Fred Sanders
- The Doctrine of The Trinity in LDS and "Catholic" Contexts, by Paul Owen
- Trinity, Stanford Encyclopedia of Philosophy
- Plantinga's View of the Trinity, by Ralph Allan Smith