Assurance, not to be confused with Eternal Security or Perseverance of the saints, is the doctrine which states
that the inner witness of the Holy Spirit in conjunction with the outward manifestation of the fruits of the Spirit allows the believer to know (be assured) that he/she is saved and
has eternal life. The apostle John's epistle's have this theme, especially his first epistle,
"I write these things to you who believe in the name of the Son of God that you may know that you have eternal life." (1 John 5:13) "Little children, let us not love in word or talk but in deed and in truth. By this we shall know
that we are of the truth and reassure our heart before him; for whenever our heart condemns us, God is greater than our heart, and he knows everything." (1 John 3:18-20) A biblical concept expressed in the writings of Augustine and emphasized by the Reformers, assurance is historically a very important doctrine among Protestants and remains so in many denominations today. In contrast, the Roman Catholic church
teaches that one may hope in his salvation, but cannot know for sure since justification is seen as a process and one's life is not yet completed.
While most Protestants believe that one can have "assurance" that they are saved at any given point in time, many such as Methodists and Lutherans believe that one's salvation can be subsequently
lost through sinful behavior or rejection of the gospel. In contrast, the Reformed tradition links the doctrine of assurance with the doctrine of Perseverance of the saints (or eternal security) which maintains
that true believers will not fall away from the faith.
- The Race Set Before Us: A Biblical Theology of Perseverance & Assurance, by Thomas R. Schreiner, Ardel B. Caneday (ISBN 0830815554)