The truth that a believer may have assurance and confidence in his salvation is rooted in the work of Christ. The believer’s confidence is not in being able to keep himself, but in the power of God and the sufficiency of Christ to keep him. This is why it is essential that the believer keep self-examination and confidence in Christ in balance — beware of emphasizing one to the exclusion of the other.
For instance, it is possible to read judgment and wrath of God passages (Jeremiah 47-51, for example), and assume that God has that same wrath against him. Yet passages like that are designed to reveal the wrath of God against the unredeemed, not the redeemed. Christ has already born the wrath of God, so that all believers are rescued from that wrath (1 Thess. 1:10).
Those who doubt their assurance also do well to remember that security does not mean perfection. While sin is forgiven and cleansed, indwelling sin still remains. Being a believer does not mean that there must be perfection of life, but that there is a particular direction of life towards God. A believer may sin, may sin repeatedly, may even sin greatly, but he does not sin completely and finally (that is, without confession and repentance).
This is why John concludes his letter with these words: “These things I have written to you who believe in the name of the Son of God, so that you may know you have eternal life” (5:13). The one who believes in Christ, demonstrates that faith through obedience and a transforming life, has assurance of that life.