...but if you don't repent you're going to hell (works)
So many pastors and teachers and laity church members admit to the core doctrine of justification by faith "alone", but then immediately whip around and plug in "...but repentance is required or else you won't be justified by faith alone".
Is this not a contradiction? Why would we need to do anything if we are justified by Faith alone? If we need to do anything else, then we are not in fact justified by faith alone, but by Faith + repentance.
It's more that continuing in sin is a sign they didn't get justified by faith, and there are varying degrees of even that doctrine. There are many pastors who will teach that yeah, okay, the bible says confess with thy mouth the Lord Jesus and believe that God raised Him from the dead and you will be saved, so.. they do that, that simple thing, they're saved, and sure, if they go on sinning they are still saved, Just God is going to make them regret every sin while they're still in the body, chastisement/scourging which there is biblical support of, Hebrews 12, and Psalm 89 for instance
Hebrews 12
5 And ye have forgotten the exhortation which speaketh unto you as unto children, My son, despise not thou the chastening of the Lord, nor faint when thou art rebuked of him:
6 For whom the Lord loveth he chasteneth, and scourgeth every son whom he receiveth.
7 If ye endure chastening, God dealeth with you as with sons; for what son is he whom the father chasteneth not?
8 But if ye be without chastisement, whereof all are partakers, then are ye bastards, and not sons.
Psalm 89
26 He shall cry unto me, Thou art my father, my God, and the rock of my salvation.
27 Also I will make him my firstborn, higher than the kings of the earth.
28 My mercy will I keep for him for evermore, and my covenant shall stand fast with him.
29 His seed also will I make to endure for ever, and his throne as the days of heaven.
30 If his children forsake my law, and walk not in my judgments;
31 If they break my statutes, and keep not my commandments;
32 Then will I visit their transgression with the rod, and their iniquity with stripes.
33 Nevertheless my lovingkindness will I not utterly take from him, nor suffer my faithfulness to fail.
and Paul also seems to teach a OSAS doctrine even among a very wicked sinner in 1 Corinthians 5
1 It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that one should have his father's wife.
2 And ye are puffed up, and have not rather mourned, that he that hath done this deed might be taken away from among you.
3 For I verily, as absent in body, but present in spirit, have judged already, as though I were present, concerning him that hath so done this deed,
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.
So here, Paul isn't saying "this guy's not saved" or that this guy has lost his salvation, but rather that he should be dealt with Church discipline, kicked out, given over to Satan to have his way with him, and then when he dies.. well, he's saved. Either drive him to a point of repentance or he dies and thus stops sinning.
However.. James 2, and 1 John is where we get the idea that sinning after profession of faith in Jesus might indicate a false faith, and they're not saved
James 2:24, and 1 John 1:6, point out that continuation of walking in sin says "they're not saved"
so it can be a difficult line to figure out.
Are you sure what they're doing is a sin, like you have scripture to declare it, not church traditions or opinions, but scripture?
although, with James 4, the bar can be said to be set so high that it's almost impossible not to be sinning daily
14 Whereas ye know not what shall be on the morrow. For what is your life? It is even a vapour, that appeareth for a little time, and then vanisheth away.
15 For that ye ought to say, If the Lord will, we shall live, and do this, or that.
16 But now ye rejoice in your boastings: all such rejoicing is evil.
17 Therefore to him that knoweth to do good, and doeth it not, to him it is sin.
For some, sin is the transgression of specific laws, but for others, sin is "missing the mark" and the mark is perfection, and for those, James is basically saying, if you're not doing the absolute best use of your time at any given time moment to moment in your life, it's sin.
Though do keep in mind that James makes a qualification here. You have to know what you should be doing, and not doing it.
and if you're sure they are sinning, are they being chastised for it or are they comfortable in it and in fact thriving and enjoying it?
Someone who sins, knows they've done wrong, and says "I repent", is someone whom Jesus said, you just keep forgiving them.
But someone who's sinning, and just.. thriving in it.. that doesn't bode well for them.