They are apostates - and by having separated themselves from the Apostolic church, they have become, in essence, idolaters. The problem with this claim of mine is that we associate the word “Idolater” strictly as someone who worships statues. But the idol is simply the visible element of the practice. The sin of idolatry is that the idol-worshipper is trying to change the relationship between God and man. God has set the rules in the form of His teaching, rites, ceremonies, sacraments, etc. These establish how He wants us to relate to Him. But the idol-worshipper does not want this, so he produces his own God(s) so he has control of the relationship.
It is not by accident that the ancient pagan pantheon of gods just happened to agree with what the humans who made them want out of life. So, there are gods of war, sex, drinking, hedonism, etc. These idol-worshippers are, therefore, not sinning - they are obeying what their gods want of them. It is wonderfully convenient - “I am not sinning when I kill, get drunk, or have illicit sex… I’m obeying the gods.” O.K., now what does this have to do with Protestantism?
The Reformation had nothing to do with “reforming” the RCC - it was about breaking away from its authority to worship as they please. Protestantism is man’s desired way to relate to Christianity. All things that are out-going or demanding are rejected: Confession to a priest, penance, purgatory, prayer for the dead, obedience to the Pope and Cardinals, celibacy, etc. In Protestantism, the formula tries to gain salvation and eternal reward as quickly, easily, and cheaply as possible. One says their sinner’s prayer and is saved. Period. No works, devotions, or charities or anything like that is ever important or required.
But this does not mean that Protestants will be damned. I believe there was a pope who stated that the Lord works His salvific purpose even in the Protestant churches. But the Protestants are exposing themselves to potential harm on the day of judgement.