Wikipedia.

A well-known symbol of Lutheranism is the so-called Luther Rose, which features a black cross on a red heart at the center. It was devised for Luther in 1530 and features multivalent symbolism. Luther claimed that:

my seal is a symbol of my theology. The first should be a black cross in a heart, so that I myself would be reminded that faith in the Crucified saves us. Although it is indeed a black cross, which mortifies and which should also cause pain, it leaves the heart in its natural color. Such a heart should stand in the middle of a white rose, to show that faith gives joy, comfort, and peace. Such a rose should stand in a sky-blue field, symbolizing that such joy in spirit and faith is a beginning of the heavenly future joy, which begins already, but is grasped in hope, not yet revealed. And around this field is a golden ring, symbolizing that such blessedness in Heaven lasts forever and has no end. 

“I wonder what symbol Calvin used?” I mused to my wife at dinner last night. “Probably a tulip,” she replied with eminent good sense. TULIP, of course, is an acronym for the five points of Calvinist theology, viz:

Total depravity
Unconditional election
Limited atonement
Irresistible grace
Perseverance of the saints

The problem is that this acronym doesn’t work in French or Latin, the two languages that Calvin operated in. Plus, the tulip may not have been introduced into Europe before Calvin’s death in 1564. 

Instead, as it turns out, Calvin did not use a flower, but a heart, held in a hand, illustrating the motto “Cor meum tibi offero, Domine, prompte et sincere,” that is, “My heart I offer to you, O Lord, promptly and sincerely.” 

The Josh Link

I’m not sure who drew this but I found it at The Josh Link. 

Calvin University.

This seventeenth-century medal was struck in memory of Calvin, and the image can be found at the Calvin University (Grand Rapids) website. 

Calvin University.

Calvin University itself uses a version of the emblem and motto. 

The more you know! Personal emblems, especially if properly heraldic, ought to make a comeback.