The main theme of The Freedom of a Christian is that the redemption and freedom of man is found through accepting Jesus Christ. In opposition to the Catholic Church and its mandates for salvation, Luther asserts that the Bible clearly establishes that people are not saved by works; they are saved by receiving the gift of God through the sacrifice of Jesus Christ, who died on the cross for their sins. Therefore, if one accepts Christ by faith, one is free from human requirements for behavior.

Another theme of this treatise is the nature of faith. In opposition to the Catholic Church and its mandates for salvation, Luther asserts that the Bible clearly establishes that people are not saved by works; they are saved by receiving the gift of God through the sacrifice of Jesus Christ, who died on the cross for their sins. Therefore, if one accepts Christ by faith, one is free from human requirements for behavior.

Luther also expounds upon the theme of the dual nature, spiritual and carnal, of man on earth. He proclaims that while man should fight the fleshly, sinful nature on earth, one is not bound by works to receive God's love and salvation. One must only receive the free gift of Christ's love. However, Luther also states that a true Christian should want to demonstrate one's love and gratitude for salvation by serving God and dying to one's sinful nature every day.

Therefore, Luther's complex statements, "A Christian is a perfectly free lord of all, subject to none. A Christian is a perfectly dutiful servant of all, subject to all," reveal the duality of a Christian's life. When Christ sets a person free from works and earning salvation, a Christian should then desire to serve God and others with a heart of gratitude and joy.