Written by Martin Luther, this specific reading talks a lot about being a "spiritual" Christian rather than a "worldly" one. He writes about the importance of how Christianity effects people's views both externally and internally. Luther emphasizes what being a Christian should truly be about, the internal.
Luther argues that the external view of Christianity does not matter in the eyes of God. Many people view Christians and judge them by how much they help others. Luther disputes that it is how a person changes through their service and Christianity that makes a difference to our salvation. He refers to this as internal Christianity, such as a person's disposition, will, heart, etc. and when it comes to doing Christian works. A person can do something nice for someone else without it bringing them joy or changing them internally. A true Christian helps others not only to help them but also because helping others brings themselves joy. Luther does not believe that worldly or external Christians will be as easily embraced in the Christian faith as spiritual or internal Christians.
I wholeheartedly agree with Martin Luther on this idea. I personally know many people who do good things for people simply for praise from others. But to me, that is not true Christianity. I go to my volunteer job to help cats that have been abandoned or abused because it makes me feel happy to see them get better, not because of the commendations that I may receive from people. I believe that I am a Christian not only because I do good works, but because doing good makes me feel good.