If they continue in a habitual unrepentant practice of sin, they are demonstrating that they are not His.
Have you ever come across a scripture in the N.T. that says that if any brother is a fornicator that he is not a brother then? All I see is the command to withdraw from fellowship with them to admonish them as brothers into repentance.
1 Corinthians 5:
11 But now I have written unto you not to keep company, if any man that is called a brother be a fornicator, or covetous, or an idolator, or a railer, or a drunkard, or an extortioner; with such an one no not to eat. 12 For what have I to do to judge them also that are without? do not ye judge them that are within? 13 But them that are without God judgeth.
Therefore put away from among yourselves that wicked person.
1 Corinthians 6:15 Know ye not that your bodies are the members of Christ? shall I then take the members of Christ, and make them the members of an harlot? God forbid. 16 What? know ye not that he which is joined to an harlot is one body? for two, saith he, shall be one flesh. 17 But he that is joined unto the Lord is one spirit.
18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body. 19 What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own? 20 For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God's.
1 Corinthians 3:10 According to the grace of God which is given unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. But let every man take heed how he buildeth thereupon. 11 For other foundation can no man lay than that is laid, which is Jesus Christ. 12 Now if any man build upon this foundation gold, silver, precious stones, wood, hay, stubble; 13 Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is. 14 If any man's work abide which he hath built thereupon, he shall receive a reward.
15 If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire. 16 Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you? 17 If any man defile the temple of God, him shall God destroy; for the temple of God is holy, which temple ye are.
So how we build on that foundation is key; if we sow to the works of the flesh, we shall reap corruption; if we sow to the fruits of the Spirit, we reap life everlasting; that life everlasting is being received as a vessel unto honor in His House or risk being a castaway for not looking to Him for help in departing from iniquity at the pre trib rapture event when God judges His House first.
2 Timothy 2:19
Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, let every one that nameth the name of Christ depart from iniquity.
20 But in a great house there are not only vessels of gold and of silver, but also of wood and of earth; and some to honour, and some to dishonour. 21 If a man therefore purge himself from these, he shall be a vessel unto honour, sanctified, and meet for the master's use, and prepared unto every good work.
So those professing believers that do not look to Him for help to depart from iniquity are the vessels unto dishonor in His House that will be left behind to face the coming fire on the earth ad the subsequent great tribulation, but they will be received later on after the great tribulation as vessels unto dishonor in His House..
1 Corinthians 5:4 In the name of our Lord Jesus Christ, when ye are gathered together, and my spirit, with the power of our Lord Jesus Christ, 5 To deliver such an one unto Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus.
Revelation 2:21 And I gave her space to repent of her fornication; and she repented not.
22 Behold, I will cast her into a bed, and them that commit adultery with her
into great tribulation, except they repent of their deeds. 23 And I will kill her children with death; and all the churches shall know that I am he which searcheth the reins and hearts: and I will give unto every one of you according to your works.
Even when a brother that has fallen away from the faith and is wicked & unreasonable, following not after the traditions taught of us and are disorderly, we are commanded to not treat them as the enemy, but withdraw from them to admonish them as brothers still in the hopes that they will be led to repent.
2 Thessalonians 3:1 Finally, brethren, pray for us, that the word of the Lord may have free course, and be glorified, even as it is with you: 2 And that we may be delivered from unreasonable and wicked men: for all men have not faith. 3 But the Lord is faithful, who shall stablish you, and keep you from evil. 4 And we have confidence in the Lord touching you, that ye both do and will do the things which we command you. 5 And the Lord direct your hearts into the love of God, and into the patient waiting for Christ.
6 Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received of us. 7 For yourselves know how ye ought to follow us: for we behaved not ourselves disorderly among you;...........14 And if any man obey not our word by this epistle, note that man, and have no company with him, that he may be ashamed.
15 Yet count him not as an enemy, but admonish him as a brother.
These are verses that clearly shows not condemning an unrepentant professing believers as if he is not our brother, but an enemy as if he was never saved.
The books of 1 John & 2 John are for guidance towards discerning those that are abiding in Him from those that are not, so new believers shall know who to have fellowship with and who to follow as an example, trusting always Jesus as their Good Shepherd to guide them through the Holy Spirit in them.
Scripture cannot go against scripture. We are not to condemn professing believers as if they were never saved. It is because they are professing Him that they are saved and why they should look to Jesus for help as their Good Shepherd to destroy the works of the devil in their lives to set them free.
John 8:30 As he spake these words, many believed on him. 31 Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed; 32 And ye shall know the truth, and the truth shall make you free. 33 They answered him, We be Abraham's seed, and were never in bondage to any man: how sayest thou, Ye shall be made free? 34 Jesus answered them, Verily, verily, I say unto you, Whosoever committeth sin is the servant of sin. 35 And the servant abideth not in the house for ever: but the Son abideth ever. 36 If the Son therefore shall make you free, ye shall be free indeed.
1 John 3:3 And every man that hath this hope in him purifieth himself, even as he is pure.....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.
1 John 1:3 That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ. 4 And these things write we unto you, that your joy may be full. 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. 6 If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth: 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. 8 If we say that we have no sin, we deceive ourselves, and the truth is not in us. 9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. 10 If we say that we have not sinned, we make him a liar, and his word is not in us.
1 John 2:1 My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous: 2 And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world.
Romans 6:10
For sin shall not have dominion over you: for ye are not under the law, but under grace.
2 Timothy 4:18 And the Lord shall deliver me from every evil work, and will preserve me unto his heavenly kingdom: to whom be glory for ever and ever. Amen.
Better to see them as the least of the brethren in need of admonishment rather than as the enemy because that is how we are taught to judge; which is to correct; not to condemn as if they were never His.