Dr. Johnson (predecessor of Norman L. Geisler and my first teacher of sound doctrine,) mentioned three assurances of salvation: The evidential (works of righteousness, love of the brethren, overcoming sin per 1 John 3:7); the internal witness of the Holy Spirit per (Rom. 8:16 as the Spirit bears witness with our spirit); and the external witness of the reliable Word of God itself. Trusting in the Word cannot be more reliable, so we should cling to our favorite verse of assurance, e.g., John 6:37 which says, "He who comes to Me I will in no wise cast out." This is like having a "spiritual birth certificate."
Salvation brings life, but assurance brings joy. "Do we have faith to die by or just one we can live with?" says Dr. Johnson.
He made it clear that one can be saved and have no assurance or a weak faith with doubts. It is God's will for us to know for sure absolutely. Not just out of curiosity, but as a boon to our faith and a duty to God to have a strong faith. Augustine of Hippo said that assurance is no "arrogant stoutness," but faith, and no presumption at all. What shall we trust: God's infallible Word or our experience? Soli Deo Gloria!
Note: Dr. Johnson baptized me and was criticized at DTS (Dallas Theological Seminary) for his strict Reformed theology. He was the pastor at Believers Chapel in Dallas, Texas. He also was one of Charles Swindoll's professors at DTS.