"For I know whom I have believed, and am persuaded that he is able to keep that which I have committed unto him against that day." (2 Timothy 1:12)
A person who thinks he is a Christian, or hopes he is a Christian, probably isn't a Christian. One should know, when it comes to this most important of all questions. "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" (1 John 5:13). It's a matter of who you know, not what you know. Paul said: "I know whom I have believed"--that is, he knew the Lord Jesus Christ, the Son of God.
But just how can we know that we have eternal life? In the first place, we know because He has said so in His word. Furthermore, He knows us! "My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand" (John 10:27, 28).
We also "know . . . that we dwell in him, and he in us, because he hath given us of his Spirit" (1 John 4:13), which "beareth witness with our spirit, that we are the children of God" (Romans 8:16). If the Holy Spirit truly has become a part of our lives (as He surely has if we genuinely came to Christ as lost sinners, trusting Him alone for forgiveness and salvation and yielding our lives and eternal souls to Him), then we will also come to love the Word which He inspired. "And hereby we do know that we know him, if we keep his commandments. . . . But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him" (1 John 2:3, 5).
There are many other things we can know, when we know we are saved. Best of all, "we know that all things work together for good to them that love God, to them who are the called according to his purpose" (Romans 8:28). HMM