How can I be sure that I have truly believed in Christ for everlasting life?

Nowhere in the Bible does God distinguish true faith from false faith. All faith is faith. If we believe in Christ for eternal life, then we know we have eternal life because Jesus guarantees it, “he who believes in Me has everlasting life,” (John 6:47). To doubt that we “truly believe” is to disbelieve Jesus’ promise. I either believe Christ’s promise or I don’t. If I do, I am saved. If I don’t, I stand condemned as one who “has not believed in the name of the only begotten Son of God” (John 3:18).

The Gospel of John does not condition eternal life on whether one “really believes” or “truly believes.” Neither does he speak of “genuine faith,” “real faith,” or “heart faith.” All faith is the conviction of the truth of some proposition. What makes saving faith saving is not the amount or uniqueness of the faith, but whom your faith is in, and what your faith believes. Saving faith believes in Jesus Christ for everlasting life. Saving faith results instantly in eternal salvation because it believes in the right object: the promise of eternal life by Jesus Christ to every believer (John 3:15-18; 6:40, 47; et al). Therefore, those who speak of “false faith” or “head faith,” are reading into faith as the Scripture neither does, nor provides basis for doing.

The Bible gives us several examples of people who knew they believed in Christ apart from good works:

(Peter) “Also we have come to believe and know that You are the Christ, the Son of the living God.” John 6:69

“Jesus heard that they had cast him [former blind man] out; and when He had found him, He said to him, ‘Do you believe in the Son of God?’ Then he said, ‘Lord, I believe!’ And he worshiped Him.” John 9:35, 38

“Jesus said to her [Martha], ‘I am the resurrection and the life. He who believes in Me, though he may die, he shall live. And whoever lives and believes in Me shall never die.  Do you believe this?’ She said to Him, ‘Yes, Lord, I believe that You are the Christ, the Son of God, who is to come into the world.’” John 11:25-27

“Then Philip said, ‘If you believe with all your heart, you may.’ And he [Ethiopian eunuch] answered and said, “I believe that Jesus Christ is the Son of God.’ ” Acts 8:37

(Paul) “For this reason I also suffer these things; nevertheless I am not ashamed, for I know whom I have believed and am persuaded that He is able to keep what I have committed to Him until that Day.” 2 Tim. 1:12