It is wonderful to have the full assurance of salvation, and it is God’s will that every one of us enjoy this assurance. Toward the close of his life the Apostle John wrote by divine inspiration:
“These things have I written unto you that believe on the name of the Son of God, that ye may know that ye have eternal life…” (I John 5:13).
There are three bases upon which believers in Christ may enjoy the full assurance of salvation: First, God urges every true believer: “Let us draw near, with a sincere heart, in full assurance of faith…” (Heb. 10:22). This is the full assurance that results from simply believing God; much as a child implicitly believes what his father has said and is absolutely sure that it is true. God says: “He that believeth on the Son hath everlasting life” (John 3:36). We may simply — and with good reason — believe His Word and enjoy the full assurance of faith.
Second, we may enjoy what Heb. 6:11 calls “the full assurance of hope.” The hope of the Bible, however, must not be confused with wishing. The Christian’s “hope” is “an anchor of the soul, both sure and stedfast” (Heb. 6:19). It comes from having proved God. Thus the full assurance of hope is the confidence that results from having accepted God’s Word.
But third, and best of all, is what Col. 2:2 calls “riches of the full assurance of understanding.” This full assurance is God’s reward to Christians who study His Word and His purposes, beginning with His plan of salvation as revealed in “the gospel of the grace of God (Acts 20:24).” When one not only believes God’s Word, but begins to understand it he cannot but be gripped by its sublime reasonableness, its powerful logic, and its provision for his deepest needs, and thus he comes to enjoy “all [the] riches of the full assurance of understanding.” source