As there is “the how” of salvation, “the how” of new life in Christ, so there is “the how” of assurance of salvation.
The way into assurance of salvation can be illustrated this way:- If a man listened to his teacher, understood everything that was taught and answered the questions that were asked in the examination correctly, the man will naturally have assurance that he has passed the examination (1Cor. 15:1-11).
In the same vein, there will be assurance of salvation naturally in the heart where there is “new life” comprising of the transformed character, new thoughts, new ambition/interest and new friendship and fellowship(1Tim. 1:12-16). This means that if one’s mind is always dwelling on things impure, untrue, immoral, dishonest, bad and ungodly, the one cannot have assurance of salvation (comp. Phil. 4:8; Isa. 32:17). Furthermore, neglecting to do some restitutions that the Lord is urging one to do through His Spirit can also cause lack of assurance of salvation.
The primary means of assurance of salvation is the witness of the indwelling Spirit of God in the heart. Once the Spirit of God is in one, the heart of such a one will be made to have assurance of salvation (Rom. 8:16; Gal. 4:6). This is done by some level of awareness or strong impression created in the heart through the indwelling Spirit of God.
Apart from the above, another means of having or retaining assurance of salvation is by believing the word of God on the remission of accidental faults (1Jn. 1:7; 2:1-2 comp. Lk. 17:3-4). If doubt is not allowed, with regard to the cleansing of these occasional faults such as bad temper, bad comments or the like, as can be found in the unsanctified, assurance of salvation will always be retained in the heart. However, to avoid the ugly effect of besetting sins, one must strive to give them up and strive for sanctification (Heb. 12:2).
Having confidence in God and believing His word implicitly enhances assurance of salvation. Also believing all doctrines of the Bible about giving and receiving, and giving oneself to doing them, establishes the heart in the Lord and promotes assurance of salvation.