How can I be sure I am a genuine Christian? The Bible tells us it is possible to know we have eternal life (1 John 5:11-13). It also says there are those who will think they know God, but Christ will say He never knew them (Matthew 7:21-23). But the Bible says we can know. Now the reason we can know is that it is not based on something we have done, but on what Christ did (Romans 5:6-8; 1 Peter 2:24,25; Ephesians 1:7). And we need to simply trust in that (Ephesians 2:8,9; Romans 4:4,5; 3:28). But as a result of our being saved, the Spirit of God works in our lives (2 Corinthians 3:18; Philippians 2:13; Colossians 1:29), resulting in a changed life (Ephesians 2:10; Titus 2:11,12; James 1:21-25).
We must not conclude that good works somehow contribute to our salvation (Galatians 2:21; Titus 3:5,6; Romans 3:20,21). If works were necessary to obtain eternal life, we could never know if we had it, because we could never know if we had done enough. But we also must not conclude that just because we went through the motions of praying a prayer or walking an aisle or assenting to a set of teachings, we have real faith, even though there was no result in our lives. Now real faith is relying on God and His promises, even for the impossible (Romans 4:16-22; Hebrews 11:1-16; 2 Corinthians 5:1-7). It is not simply a knowledge of the facts, which even the demons have (James 2:19). This genuine faith will result in a changed life (Matthew 7:16-20; James 2:14-26; Hebrews 12:14).
Now we do not want to deceive people into believing they are Christians if they are not. But we also need to avoid making the standard so high that no honest person could ever really know they have eternal life. One useful distinction I have found is the Lot criterion. If we look at the story of Lot in the Old Testament we do not see a picture of perfect moral behavior (Genesis 13-19). Lot selfishly chooses the best land, lives there in spite of the moral decadence of the inhabitants, offers his daughters to be raped to protect his guests, and ends up committing incest with his daughters in a drunken stupor. Nonetheless, 2 Peter 2:7,8 says Lot was a righteous man (he was saved). But it also says he was tormented in his soul over the deeds done in Sodom. He was able to live in Sodom, but not to be at home in Sodom. Ultimately, only we can examine ourselves before God (2 Corinthians 13:5) and decide if we have a genuine faith which causes us not to feel at home in sin. Not that we should just settle for passing the Lot criterion, but we should press on to grow in Christ (Philippians 3:12-16; Hebrews 12:1-3; Ephesians 4:12-16), that our faith may be more clearly seen by what it produces in our lives.