This is an important doctrine upon which a great deal depends, that we regard our calling as comprehended in God’s Word, so that each one may be sure that all he does and leaves undone he does and leaves undone in the name and at the command of God. Thus the soul is prepared to defy the devil, can be of good cheer, and say: I have today done this and that, and I have done it, because I know God has told me and commanded in His Word to do so; I therefore know that it is a good work with which God is well pleased. Who ever cannot speak thus should abandon his work, no matter what it may be; for it is sin and God is displeased with it, as St. Paul saith: “Whatsoever is not of faith is sin.”
Here we must know, that whoever does not want to live in uncertainty, like the heathen, but desires so to govern his actions that God may be pleased and to do all in faith, must have and know the Word of God. For faith cannot exist where the Word of God is not. You may wonder what such Word is, that comprehends all vocations and our whole external life. But we can easily tell you. In the first place, there is the fourth commandment: “Honor thy father and thy mother.” This commandment embraces all stations of life. For the state of matrimony is the source of all other states. God has made one state depend upon the other; as the state of matrimony must serve all others — for no civil government nor civil well being could exist without it — so again all other states have to serve the state of matrimony. (Luther’s House Postil, Sermon for the Fifth Sunday after Trinity, Volume 3, p. 697)