Why do you think he kept infant baptism?
I think Luther realized that no oath nor action on our parts can save us. While Baptism is not a salvation issue among Protestants, it certainly is for Catholics who must be baptized into the Holy Roman Catholic Church in order to be saved.
All men are told to obey God. Some do; some don't. We're told to repent, believe, be baptized, perform good works, love God...all of which is the Holy Spirit working within us.
And just like the "wind and the sea obey Him" because they are compelled and have no choice, all those whom God ordains to save will obey, repent, be baptized, perform good works and love God.
Salvation is of the Lord. Christ will lose none of His sheep whom God has given Him to lead home. Christianity is a specific, particular faith. God breathed your name among His flock from before the foundation of the world.
Baptism confirms that fact.