A Christian may KNOW that he is in a state of grace, that he shall go to Heaven:
“We KNOW that we have passed from death to life” (1 John 3:14)
“We KNOW that if our earthly tabernacle were dissolved, we have a building of God, a house not made with hands, eternal in the heavens” (2 Corinthians 5:1) The Lord does not leave His people at uncertainties in matters of salvation. The apostle says “we KNOW that all things work together for good to them that love God” (Romans 8:28) It is not a matter wavering or doubt. Hence observe that the truths of the gospel are evident and infallible. We have arrived at a holy confidence. We have both the spirit of God, and our own experience, setting seal to it. “The Spirit Himself testifies with our spirit that we are children of God” (Romans 8:16) “I write these things to you who believe in the name of the Son of God, so that you may KNOW that you have eternal life” (1 John 5:13)
(Thomas Watson)