I read John 8: 34-36 as referring to unbelievers only - as both believers and unbelievers will still commit sin – so who is the slave…? Perhaps I need to read John 8: 34-36 in ‘context’ – but seeing it quoted ‘alone’ is a one or the other prospect.
Anyone and everyone who sins.
Joh 8:34 Jesus answered them, Verily, verily, I say unto you,
Whosoever committeth sin is the servant of sin.
Though Jesus was speaking to the Jews at the time, "Whosoever" back then means whosoever today, even as sin back then is still sin today, in the eyes of God.
Paul gives a detailed account of the Christian who is a servant/slave to sin and has not yet been freed from the cords of sin.
Rom 6:12
Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof.
Rom 6:13
Neither yield ye your members as instruments of unrighteousness unto sin: but yield yourselves unto God, as those that are alive from the dead, and your members as instruments of righteousness unto God.
Rom 6:14
For sin shall not have dominion over you: for ye are not under the law, but under grace.
Rom 6:15 What then?
shall we sin, because we are not under the law, but under grace? God forbid.
Rom 6:16
Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?
Rom 6:17 But God be thanked, that
ye were the servants of sin, but ye have obeyed from the heart that form of doctrine which was delivered you.
Rom 6:18 Being then made free from sin, ye became the servants of righteousness.
The book of Romans was written to Christian and is therefore addressing the same. If Paul tell them to not allow sin to reign in their bodies, then it is very possible for Christian to be servants of sin. As it is stated below.
Rom 7:15
For that which I do I allow not: for what I would, that do I not; but what I hate, that do I.
Rom 7:16
If then I do that which I would not, I consent unto the law that it is good.
Rom 7:17 Now then
it is no more I that do it, but sin that dwelleth in me.
Rom 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.
Rom 7:19
For the good that I would I do not: but the evil which I would not, that I do.
Rom 7:20 Now
if I do that I would not, it is no more I that do it, but sin that dwelleth in me.
Rom 7:21 I find then a law, that, when I would do good, evil is present with me.
Rom 7:22 For I delight in the law of God after the inward man:
Rom 7:23 But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members.
Rom 7:24 O wretched man that I am! who shall deliver me from the body of this death?
Rom 7:25 I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh the law of sin.
One cannot be free from sin if they have not been taught that they have been freed in Christ Jesus and act on that word, because it is the word of God itself that makes them free.
If you have not received that part of the gospel of Jesus' work on the cross, then the born again child of God can and will remain a servant of the sin. And one cannot be set free from that sin UNTIL they repent of it and confess it before God. THEN and only then will God forgive them of that sin and set them free.
We are not automatically forgiven, even as we are not automatically saved when we become adults. Even grace comes with conditions.
What might that be, you ask.
You must be born again through faith to enter into the grace of God. The unbeliever is not under or in the grace of God, the believer is.