Babylonian Captivity of the Church:
Therefore I advise no one to enter any religious order or the priesthood, indeed, I advise everyone against it - unless he is forearmed with this knowledge and understands that the works of monks and priests, however holy and arduous they may be, do not differ one whit in the sight of God from the works of the rustic laborer in the field or the woman going about her household tasks, but that all works are measured before God by faith alone. . . . Indeed, the menial housework of a manservant or maidservant is often more acceptable to God than all the fastings and other works of a monk or priest, because the monk or priest lacks faith. since, therefore, vows nowadays seem to tend only to the glorification of works and to pride, it is to be feared that there is nowhere less of a faith and of the church than among the priests, monks, and bishops. These men are in truth heathen or hypocrites. They imagine themselves to be the church, or the heart of the church, the "spiritual" estate and the leaders of the church, when they are everything else but that. This is indeed "the people of the captivity," among whom all things freely given to us in baptism are held captive, while the few poor "people of the earth" who are left behind, such as the married folk, appear vile in their eyes.
Lectures on Genesis 26-50:
What you do in your house is worth as much as if you did it up in heaven for our Lord God. For what we do in our calling here on earth in accordance with His word and command He counts as if it were done in Heaven for Him. . . .
Therefore we should accustom ourselves to think of our position and work as sacred and well-pleasing to God, not on account of the position and the work, but on account of the word and faith from which the obedience and the work flow. No Christian should despise his position and life if he is living in accordance with the word of God, but should say, "I believe in Jesus Christ, and do as the ten commandments teach, and pray that our dear Lord God may help me thus to do." That is a right and holy life, and cannot be made holier even if one fast himself to death.
. . . It looks like a great thing when a monk renounces everything and goes into a cloister, carries on a life of asceticism, fasts, watches, prays, etc. . . . On the other hand, it looks like a small thing when a maid cooks and cleans and does other housework. But because God's command is there, even such a small work must be praised as a service to God far surpassing the holiness and asceticism of all monks and nuns. For here there is no command of God. But there God's command is fulfilled, that one should honour father and mother and help in the care of the home.
Exposition of Psalm 128
Your work is a very sacred matter. God delights in it, and through it he wants to bestow his blessings on you. This praise of work should be inscribed on all tools, on the forehead and faces that sweat from toiling.
The Estate of Marriage
Now you tell me, when a father goes ahead and washes diapers or performs some other mean task for his child, and someone ridicules him as an effeminate fool, though that father is acting in the spirit just described and in Christian faith, my dear fellow you tell me, which of the two is most keenly ridiculing the other? God, with all his angels and creatures, is smiling, not because that father is washing diapers, but because he is doing so in Christian faith. Those who sneer at him and see only the task but not the faith are ridiculing God with all his creatures, as the biggest fool on earth. Indeed, they are only ridiculing themselves; with all their cleverness they are nothing but devil's fools.
See also Martin Luther's An Open Letter to the Christian Nobility