People who deny eternal security think it’s unfair to say that Christians can sin and get away with it. 

But that’s a misunderstanding. 

The doctrine of eternal security doesn’t say you get away with sin. All it means is that whatever the consequences may be, they do not include losing your eternal salvation (see here and here and  here).   

The warnings are about some kind of potential loss, but what? 

Michael Eaton, who was a Free Grace theologian in the “thin” sense of the term, explains all the things you can lose when you sin:  

Sin is forgivable and yet John does not want us to sin. If we Christians do sin we shall be forgiven but we shall lose a lot: our peace, our joy, our good conscience, our usefulness to God, eventually our reputation, our ministry. We shall lose our treasure in heaven. We are in danger of experiencing God’s chastening. Our life could be lost. We will be saved through fire, which is the worst kind of mixture of blessing-and-tragedy. God is altogether against our sinning and we cannot go against God and it turn out good (1, 2, 3 John & Jude, p. 66). 

Believing in eternal security does not mean sanctioning sin. On the contrary, it can help you see the destructiveness of sin more clearly.