Legalism is man trying to be saved through his good works, while Biblical holiness is one who is already saved, showing good works as a result of their salvation (James 2:26), and never the means of ones salvation (Ephesians 2:8-9)! – Evangelist Sean Bonitto
Likewise with rock music. A legalistic Christian would stay away from it because it is the devil’s music. He may think that God will not be pleased with him if he listens, hence trying to gain approval of God. While the saint of God is aware of the history and the occult source of the music, realising that there is no middle ground between God and Satan and hence chooses to be on God’s side. Again, he intention is to obey and revere the command of God of being holy as He is holy. So what is wrong with rock music? Well to begin with. the whole world listens to it and is under its influence, where is the setting apart of the believer in that? (Plus rock has many side-effects: personal as well as societal (the spiraling of morals began in the 1960s, around the same time rock started becoming popular. I would encourage any rock-lover (even with “christian” lyrics) to do some research on it)
I hope you are able to get the subtle yet huge difference between legalism and biblical holiness. It’s all in the motive behind the refraining: you are legalistic if you do things because you want to be saved and appease God, but you are holy by the biblical standards if your good works are a result of the changed life from making Christ your Lord and obeying His command to be set apart from the world. There is no room for works of self-rightousness when we turn to the Lord, but only His blood that makes us righteous.
Denying ungodliness and worldly lusts, we should live soberly, righteously, and godly in this present world – Titus 2:12