When a person becomes a Christian by trusting Jesus as their saviour, they are given a brand new heart and God puts his Holy Spirit in their heart. This is called the new man. When a person is filled with the Holy Spirit (letting the Holy Spirit lead him and guide him), he wants to do right and please God in everything he says and does and he wants to glorify God. The problem is, that even though we have this new man, we still have the old man too, and that is why we sin.
II 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.
Ephesians 4:24: And that ye put on the new man, which after God is created in righteousness and true holiness.
Colossians 3:9: Lie not one to another, seeing that ye have put off the old man with his deeds;
10: And have put on the new man, which is renewed in knowledge after the image of him that created him.
The old man is also called walking in the flesh. The new man walks in the Holy Spirit. Galatians 5:16: This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh.
In Romans chapter 7 there is a section which explains why we sin even when we are wanting to do good. There is a struggle to do right because we have the old man and the new man and they fight against each other.
Romans 7:18: For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.
19: For the good that I would I do not: but the evil which I would not, that I do.
20: Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.
In the book of I John there are many passages about sin. Some of the verses that say we cannot sin are talking about when we are walking in the spirit. For instance, I John 3:6: Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him. When we are walking in the spirit we do not sin, but the problem is we are not always walking in the spirit even though we are always saved. We still have the old man in us, and the bible says if we say we are not a sinner then we are a liar.
I John 1:8: If we say that we have no sin, we deceive ourselves, and the truth is not in us. (Deceived means believing a lie).
I John 1:10: If we say we have not sinned, we make him a liar, and his word is not in us.
So what should we do when we sin? The answer is in I John 1:9: If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
I John 1:7: But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.
I John 2:12: I write unto you, little children, because your sins are forgiven you for his name’s sake.
Proverbs 28:13: He that covereth his sins shall not prosper: but whoso confesseth and forsaketh them shall have mercy.
Luke 7:47: Wherefore I say unto thee, Her sins, which are many, are forgiven; for she loved much: but to whom little is forgiven, the same loveth little.
7:48: And he said unto her, Thy sins are forgiven.
The difference between a saved person and an unsaved person is that an unsaved person is just totally dead in sin because they don’t even have the Holy Spirit, they have never been forgiven of anything. Ephesians 2:1: And you hath he quickened, who were dead in trespasses and sins:
