Martin Luther explained the symbolism of his seal in a letter to Lazarus Spengler (July 8, 1530):
Honorable, kind, dear Sir and Friend!
Since you ask whether my seal has come out correctly, I shall answer most amiably and tell you of those thoughts which now come to my mind about my seal as a symbol of my theology.
There is first to be a cross, black and placed in a heart, which should be of its natural color, so that I myself would be reminded that faith in the Crucified saves us. For if one believes from the heart he will be justified. Even though it is a black cross, which mortifies and which also should hurt us, yet it leaves the heart in its natural color and does not ruin nature; that is, the cross does not kill but keeps man alive. For the just man lives by faith, but by faith in the Crucified One.
Such a heart is to be in the midst of a white rose, to symbolize that faith gives joy, comfort, and peace; in a word it places the believer into a white joyful rose; for this faith does not give peace and joy as the world gives and, therefore, the rose is to be white and not red, for white is the color of the spirits and of all the angels.
Such a rose is to be in a sky-blue field, symbolizing that such joy in the Spirit and in faith is a beginning of the future heavenly joy; it is already a part of faith, and is grasped through hope, even though not yet manifest. And around this field is a golden ring, symbolizing that in heaven such blessedness lasts forever and has no end, and in addition is precious beyond all joy and goods, just as gold is the most valuable and precious metal.
May Christ, our dear Lord, be with your spirit until the life to come.
Source: Luther’s Works, vol. 49: Letters II, ed. Jaroslav Jan Pelikan, Hilton C. Oswald, and Helmut T. Lehmann (Fortress, 1999), 358–359.