What is Christian freedom?

Luther:

A Christian is a free lord about all things and subject to none; but a Christian is also a servant in all things and everyone’s subject.

That seems to be contradictory, but both are true. Christ too was both free and slave at the same time: nobody could impose on Him, but He serves everybody. It is as Paul says in 1 Corinthians 9 that he makes himself a servant for everybody, although he is free, and in Romans 13 that we should owe noone anything than love.

Faith makes us free, love makes us servants. That is the sum of it.