There are three main grounds of assurance of salvation, each beginning with W.
The Word of the Father
The Work of Christ
The Witness of the Spirit
The first two Ws are quite straightforward:
The Word of the Father promises salvation to those who believe in the Son. We can rest on these biblical promises because God does not lie (Titus 1:2). His word is dependable. John 3:16 is one example of a promise that ought to give believers great assurance of salvation.
The Work of Christ on the cross provides salvation. Isaiah 53:5-6 teaches believers that through the cross, “the punishment that brought us peace was upon him.” There is nothing left to pay. It is rather insulting to the cross when downhearted believers wonder whether they are too guilty to be saved.
The third W needs more explanation:
The Witness of the Spirit is our experience of the reality of the indwelling Spirit, who preserves salvation. The logic flows like this: If you experience the Spirit’s power in your life, helping you to resist sin and live for God, you can be confident that you have the Spirit (Romans 8:6-9). Knowing that you have the Spirit reassures you that you will be saved because he is given to preserve our faith until the end: “Having believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession — to the praise of his glory” (Ephesians 1:13-14).
Another way to think about the witness of the Spirit is to examine your opinion of Jesus. If you see in Jesus the glory of God (2 Corinthians 4:4-6), and if the sight of that glory has stirred up some measure of Christlikeness in your life (2 Corinthians 3:18), then the Spirit testifies that you have eternal life, because it’s only by the Spirit that people truly believe in Jesus as God’s Son (see 1 John 5:6-11). This is what J.I. Packer has in mind when he says that the internal witness of the Spirit is:
a work of enlightenment whereby, through the medium of verbal testimony … blind eyes … are opened, and divine realities come to be recognized and embraced for what they are. This recognition … is as immediate and unanalysable as the perceiving of a colour, or a taste, by physical sense – an event about which no more can be said than that when appropriate stimuli were present it happened, and when it happened we knew it had happened.
(Emphasis added. From “Calvin the Theologian” via http://www.desiringgod.org/biographies/the-divine-majesty-of-the-word)
Meditating on the three Ws is a lifetime’s task but the more we engage in that task the more assurance of salvation we will enjoy.