Certainty of salvation, because of the promises of God and the effectiveness of Christs atonement (1 John 5:13).
The word does not occur often in the Bible, but the idea is more frequent. It is basic that people do not deserve their salvation because of their own efforts; that would leave them always uncertain, never knowing whether they had been good enough. But Christ did all that was needed, and we can rely on his perfect work. Further, believers have evidence of Gods power in their lives (1 John 2:3-5; 3:19-21).
Our assurance rests on the certainty that what God has begun he will complete (Phil. 1:6).