The Word of God teaches that the new birth is essential and that "The Spirit itself beareth witness with our spirit, that we are the children of God"  (Romans 8:16).  Being born again is like becoming a semi-detached house - I have my old nature (very strong) and my new nature (new born).  The old nature must be crucified/mortified by my setting my will, God's grace, and by and through His power; so that the new nature can grow.  Having two natures is like living in an 'enemy body'.
John 3:3:
"Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God."
2 Corinthians 5:17:
"Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new."
Romans 6:6:
"Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin."
Galatians 2:20:
"I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me."
Galatians 5:24:
 "And they that are Christ's have crucified the flesh with the affections and lusts."
Romans 8:13:
"For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live."
Colossians 3:5:
"Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry:"
Romans 12:1:
"I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service."