Nevertheless the foundation of God standeth sure,
having this seal,
The Lord knoweth them that are his.
Let every one that nameth the name of Christ depart from iniquity.
Here in 2 Timothy 2:19 the Apostle Paul gives the ground rules for discerning whether or not I am a Christian.
The first rule is that salvation is not a one-sided transaction – it is not enough to say that I know God and that from now on I will be a Chriustian. The real question is “Does God know me?” Jesus warned that at the judgment there will be many who claim that they know Him and did many wonderful things in service to Him, yet He will reply “I never knew you.”
God calls, I respond, and if God sees that I have responded with all my heart, no reservations, he adopts me as His child. We will know when this happens. “For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father” (Romans 8:15). “And hereby we know that he abideth in us, by the Spirit which he hath given us” (1 John 3:24).
The second rule is that I cannot go on living as I did before. “And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them” (Ezekiel 36:27). “He that saith he is in the light, and hateth his brother, is in darkness even until now. He that loveth his brother abideth in the light, and there is none occasion of stumbling in him” (1 John 2:9-10) .
Theology can quickly become very complex, and sometimes it is merely an intellectual exercise that offers no real help in showing us how to truly live as a Christian. But if I have the Holy Spirit within me and love my neighnour as myself, I will not want to steal, lie, cheat or hurt anyone. Why make it more complicated than that?