If we were to explain what one thing makes someone a Christian, we would shout, “A NEW NATURE!” We can’t fool God.
Jesus answered and said to him, “Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.” Nicodemus said to Him, “How can a man be born when he is old? He cannot enter a second time into his mother’s womb and be born, can he?” Jesus answered, “Truly, truly, I say to you, unless one is born of water and the Spirit he cannot enter into the kingdom of God. “That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. “Do not be amazed that I said to you, ‘You must be born again.’ John 3:3-7
No matter how proficient someone gets at being good – no matter how accomplished someone becomes at being evil – it is all from the same source: the old nature. The human race tries again and again to dress up the old nature with religion, positive thinking, cultic worship, etc., etc., but it is still what it always was – a nature whose end is death.
“I am the living bread that came down out of heaven; if anyone eats of this bread, he will live forever; and the bread also which I will give for the life of the world is My flesh.” “This is the bread which came down out of heaven; not as the fathers ate and died; he who eats this bread will live forever.” John 6:51 and 6:58
For neither is circumcision anything, nor uncircumcision, but a new creation. Galatians 6:15
New creatures are not the same as the old. They have within their new nature all the characteristics of that new nature.
In regards to the mixing of the old and new nature, Jesus was very protective of His disciples. The mixing of the two natures was not His intention for them or for us:
Then the disciples of John *came to Him, asking, “Why do we and the Pharisees fast, but Your disciples do not fast?” And Jesus said to them, “The attendants of the bridegroom cannot mourn as long as the bridegroom is with them, can they? But the days will come when the bridegroom is taken away from them, and then they will fast. But no one puts a patch of unshrunk cloth on an old garment; for the patch pulls away from the garment, and a worse tear results. Nor do {people} put new wine into old wineskins; otherwise the wineskins burst, and the wine pours out and the wineskins are ruined; but they put new wine into fresh wineskins, and both are preserved.” Mark 9:14-17
To attempt to “look or act” like a Christian is not possible. The old cloth will tear and the old skin will burst. New must be new. We can’t fool God.