By the fruit a tree puts forth, you will recognize the tree. A good tree does not put forth bad fruit, just as a bad tree does not put forth good fruit. By the fruit of the life of the one who professes to be a Christian (their actions) you will recognize them.1
All who profess to be a Christian are not necessarily a Christian.2 All who are truly Christian, however, are eternally secure in their relationship with their Heavenly Father. God will never leave, nor forsake, them.3 They have an inheritance, reserved for them in heaven, that can never perish, spoil, or fade.4
The Bible tells us that we are saved by faith alone, but faith that does not produce good works is dead faith.5 A child of God does sin from time to time, and when they do, God has said that if we confess (or agree) with Him that we have sinned and turn away from it, that He will forgive us our sin and the blood of Jesus Christ, which was shed on the cross of Calvary as atonement of our sin debt, continues to purify us from all unrighteousness.6
To those who claim to be a child of God who, however, practice sin (live in sin with no conviction from the Holy Spirit or any desire to forsake it) they need to be aware of what the apostle John wrote in the New Testament of the Bible under inspiration of the Holy Spirit. The Bible says this:
"How great is the love the Father has lavished on us, that we should be called children of God! And that is what we are! The reason the world does not know us is that it did not know him. Dear friends, now we are children of God, and what we will be has not yet been made known. But we know that when He appears, we shall be like Him, for we shall see Him as He is. Everyone who has this hope in him purifies himself, just as He is pure.
Everyone who sins breaks the law; in fact, sin is lawlessness. But you know that He appeared so that He might take away our sins. And in Him is no sin. No one who lives in Him keeps on sinning. No one who continues to sin has either seen Him or known Him.
Dear children, do not let anyone lead you astray. He who does what is right is righteous, just as He is righteous. He who does what is sinful is of the devil, because the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the devil's work. No one who is born of God will continue to sin (practice sin) because God's seed remains in him; he cannot go on sinning, because he has been born of God. This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not a child of God; nor is anyone who does not love his brother."7
1 (Matthew 7:15-20)
2 (Matthew 7:21-23)
3 (Hebrews 13:5)
4 (1 Peter 1:3-5)
5 (James 2:14-18)
6 (1 John 1:9; Romans 8:1)
7 (1 John 3:1-10 NIV, words in parenthesis are mine)