Paul wrote this to the Romans...
Rom 16:17 Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them.
Rom 16:18 For they that are such serve not our Lord Jesus Christ, but their own belly; and by good words and fair speeches deceive the hearts of the simple.
Rom 16:19 For your obedience is come abroad unto all men. I am glad therefore on your behalf: but yet I would have you wise unto that which is good, and simple concerning evil.
Jude warned that...
Jud 1:3 Beloved, when I gave all diligence to write unto you of the common salvation, it was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints.
Jud 1:4 For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ.
These deceivers attack the grace message and turn it into a license to sin. So when Paul teaches this...
Rom 6:1 What shall we say then? Shall we continue in sin, that grace may abound?
Rom 6:2 God forbid. How shall we, that are dead to sin, live any longer therein?
They deny it and twist scriptures like this...
1Jn 1:8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
...to prove that a Christian exists in an ongoing state of sinfulness and that those who deny it are liars.
When...
1Joh 1:8-10 is actually in the context of this...
1Jn 1:5 This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all.
1Jn 1:6 If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth:
1Jn 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.
So John continues with an explanation of HOW to walk in the light.
1Jn 1:8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
1Jn 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
1Jn 1:10 If we say that we have not sinned, we make him a liar, and his word is not in us.
A Parallel verse in the Old Testament is this...
Pro 28:13 He that covereth his sins shall not prosper: but whoso confesseth and forsaketh them shall have mercy.
A parallel from the teachings of Jesus is this...
Luk 15:11 And he said, A certain man had two sons:
Luk 15:12 And the younger of them said to his father, Father, give me the portion of goods that falleth to me. And he divided unto them his living.
Luk 15:13 And not many days after the younger son gathered all together, and took his journey into a far country, and there wasted his substance with riotous living.
Luk 15:14 And when he had spent all, there arose a mighty famine in that land; and he began to be in want.
Luk 15:15 And he went and joined himself to a citizen of that country; and he sent him into his fields to feed swine.
Luk 15:16 And he would fain have filled his belly with the husks that the swine did eat: and no man gave unto him.
Luk 15:17 And when he came to himself, he said, How many hired servants of my father's have bread enough and to spare, and I perish with hunger!
Luk 15:18 I will arise and go to my father, and will say unto him, Father, I have sinned against heaven, and before thee,
Luk 15:19 And am no more worthy to be called thy son: make me as one of thy hired servants.
Luk 15:20 And he arose, and came to his father. But when he was yet a great way off, his father saw him, and had compassion, and ran, and fell on his neck, and kissed him.
Luk 15:21 And the son said unto him, Father, I have sinned against heaven, and in thy sight, and am no more worthy to be called thy son.
Luk 15:22 But the father said to his servants, Bring forth the best robe, and put it on him; and put a ring on his hand, and shoes on his feet:
Luk 15:23 And bring hither the fatted calf, and kill it; and let us eat, and be merry:
Luk 15:24 For this my son was dead, and is alive again; he was lost, and is found. And they began to be merry.
If 1Joh 1:8-9 was a verse contending for an ongoing state of sinfulness in the life of a Christian then John would not have made the following statements...
1Jn 3:6 Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him.
1Jn 3:7 Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous.
1Jn 3:8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
1Jn 3:9 Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
1Jn 3:10 In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.
1Jn 5:18 We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not.