Without commentary (let there be commentary in the comments section)...
To His Serene Highness, Prince John Frederick, Duke Of Saxony, Landgrave Of Thuringia, Margrave Of Meissen, My Gracious Lord And Patron SERENE and high-born Prince, gracious Lord! May your Grace accept my humble prayer and service.
Your Grace’s kind letter has lately come into my hands and its cheering contents brought me much joy. By way of reply I send you this little exposition of the Magnificat, which I long since promised you, but which the troublesome quarrels of many adversaries have repeatedly interrupted.
If I put it off any longer I shall have to blush for shame; nor is it meet that I make further excuses, lest your Grace’s youthful spirit should be retarded, which inclines to love of Holy Writ, and which by further exercise in the same might be the more stirred up and strengthened. To which end I wish your Grace God’s grace and help.
And of this there is sore need. For the welfare of many lies in the power of so mighty a prince, if he be taken out of himself and graciously ruled by God, just as, on the other hand, the ruin of many lies in his power, if he be left to himself and ruled by God’s disfavor. For while the hearts of all men are in God’s almighty hand, it is not without reason said of kings and princes alone, “The king’s heart is in the hand of the Lord: he turneth it whithersoever he will.” Whereby God would instill His fear in the mighty lords and teach them that all their thoughts and intentions are naught without His special inspiration. Other men’s acts bring gain or loss upon themselves alone or upon but few others; but rulers are appointed for the particular purpose of being either harmful or helpful to other men only,-and to the more men, the wider their domains. Wherefore the Scripture also calls pious and God-fearing princes angels of God and even gods; but harmful princes it calls lions, dragons and wild beasts, which God includes amongst His four plagues — pestilence, famine, war, and noisome beasts. The heart of man, then, being by nature but flesh and blood, is of itself prone to presumption; and when, besides this, power, riches and honor fall to his lot, these form so strong an incentive to presumption and overconfident security as to move him to forget God and despise his subjects. Being able to do wrong with impunity, he lets himself go and becomes a beast, does whatever he pleases, and is a ruler in name, but a monster in deed. Wherefore the sage Bias has well said, Magistratus virum ostendit, — the office of ruler reveals what manner of man the ruler is. As for the subjects, they dare not let themselves go for fear of the authorities. Therefore all rulers, since they need not fear men, should fear God more than others do, should learn to know Him and His works, and walk diligently, as St. Paul says in Romans 12:8, “He that ruleth, let him do it with diligence.”
Now I do not know in all the Scriptures anything that so well serves such a purpose as this sacred hymn of the most blessed Mother of God, which ought indeed to be learned and kept in mind by all who would rule well and be helpful lords. Truly she sings in it most sweetly of the fear of God, what manner of lord He is, and especially what His dealings are with those of high and of low degree. Let another listen to his love singing a worldly ditty; this pure Virgin well deserves to be heard by a prince and lord, as she sings him her sacred, chaste and salutary song. It is a fine custom, too, that this canticle is sung in all the churches daily at vespers, and to a particular and appropriate setting that distinguishes it from the other chants. May the tender Mother of God herself procure for me the spirit of wisdom, profitably and thoroughly to expound this song of hers, so that your Grace as well as we all may draw therefrom wholesome knowledge and a praiseworthy life, and thus come to chant and sing this Magnificat eternally in heaven. To this may God help us. Amen.
Herewith I commend myself to your Grace, humbly praying your Grace in all kindness to receive my poor effort.
Your Grace’s obedient chaplain, DR. MARTIN LUTHER.
Wittenberg, 10 March, 1521. (source: http://www.godrules.net/library/luther/NEW1luther_c5.htm)