It is to bow the head, bend the body, fall on the knees, prostrate one's self, and so forth, and to do such things as a sign and acknowledgment of an authority and power; just as people bow in silence before secular princes and Lords, and just as popes, bishops, abbots, and people generally, have themselves honored and adored [ehrbieten] by bowing and kneeling, and so forth. Such outward adoration [ehrbietunge] is what the Scriptures really mean by worship [anbeten].... We read in the Scriptures that worship [anbeten] or adoration [ehrbieten] is rendered outwardly both to God and to kings without distinction, just as bowing and kneeling are still rendered outwardly both to God and to men.
From this understanding of outward worship you will also understand what Christ meant by true spiritual worship. It is the adoration or bowing of the heart, so that from the bottom of your heart you thereby show and confess yourself to be his subordinate creature. For from this you see that true worship can be nothing else than faith; it is faith's sublimest activity with respect to God. For no one is capable of such heartfelt confession, adoration, bending, and bowing (or whatever you want to call it) before God in his heart, unless he unwaveringly holds God to be his Lord and Father, from whom he receives and will receive all good things, and through whom, without any merit on his part, he is redeemed and preserved from all sins and evil.