Good point, there are probably many saints who don't know what an apostate is.
4 For certain individuals whose condemnation was written about long ago have secretly slipped in among you. They are ungodly people, who pervert the grace of our God into a license for immorality and deny Jesus Christ our only Sovereign and Lord.
They secretly slip into the church, into your house, kind of like Santa coming down the chimney. They try to pervert the grace of God to materialism so that your prayers become a list of things you want from God, like the letter they write to Santa. They have parties that become a license for immorality. And they deny Jesus Christ, X'ing out His name.
17 But, dear friends, remember what the apostles of our Lord Jesus Christ foretold. 18 They said to you, “In the last times there will be scoffers who will follow their own ungodly desires.” 19 These are the people who divide you, who follow mere natural instincts and do not have the Spirit.
A scoffer refers to scorn, mocks or derides. For example, look at post #202, that is what it means to deride.
What is the desire behind all of the mythology in Christmas with the tree, stockings, and Santa? It is ungodly. It is also divisive. There are those who want to be absolute for the Lord, to follow the apostles fellowship and they are attacked, mocked and ridiculed because they don't adopt these pagan practices that have their origin in idolatry.