Jesus tells us that we cannot serve two masters (Matt. 6:24). Either you are all in with God and Jesus, or you are still living for your sinful flesh desires. God tells us in the Bible how to know if we are truly followers of Christ.
And how can we be sure that we belong to him? By looking within ourselves: are we really trying to do what he wants us to? Someone may say, “I am a Christian; I am on my way to heaven; I belong to Christ.” But if he doesn’t do what Christ tells him to, he is a liar. But those who do what Christ tells them to will learn to love God more and more. That is the way to know whether or not you are a Christian. Anyone who says he is a Christian should live as Christ did. (1 John 2:3-6, TLB)