But the holy orders and true religious institutions established by God are these three: the office of priest, the estate of marriage, the civil government. All who are engaged in the clerical office or ministry of the Word are in a holy, proper, good, and God-pleasing order and estate, such as those who preach, administer sacraments, supervise the common chest, sextons and messengers or servants who serve such persons. These are engaged in works which are altogether holy in God's sight.
Again, all fathers and mothers who regulate their household wisely and bring up their children to the service of God are engaged in pure holiness, in a holy work and a holy order. Similarly, when children and servants show obedience to their elders and masters, here too is pure holiness, and whoever is thus engaged is a living saint on earth.
Moreover, princes and lords, judges, civil officers, state officials, notaries, male and female servants and all who serve such persons, and further, all their obedient subjects – all are engaged in pure holiness and leading a holy life before God. For these three religious institutions or orders are found in God's Word and commandment; and whatever is contained in God's Word must be holy, for God's Word is holy and sanctifies everything connected with it and involved in it.
Above these three institutions and orders is the common order of Christian love, in which one serves not only the three orders, but also serves every needy person in general with all kinds of benevolent deeds, such as feeding the hungry, giving drink to the thirsty, forgiving enemies, praying for all men on earth, suffering all kinds of evil on earth, etc. Behold, all of these are called good and holy works. However, none of these orders is a means of salvation. There remains only one way above them all, viz. faith in Jesus Christ.
For to be holy and to be saved are two entirely different things. We are saved through Christ alone; but we become holy both through this faith and through these divine foundations and orders. Even the godless may have much about them that is holy without being saved thereby. For God wishes us to perform such works to his praise and glory. And all who are saved in the faith of Christ surely do these works and maintain these orders.
What was said about the estate of marriage, however, should also be applied to widows and unmarried women, for they also belong to the domestic sphere.
Martin Luther, Confession Concerning Christ's Supper, quoted in Martin Luther's Basic Theological Writings (ed. Timothy Lull), pp.54f.