In fact, the distinction between latria and dulia, as they called them, was invented in order that divine honors might seem to be transferred with impunity to angels and the dead. For it is obvious that the honor the papists give to the saints really does not differ from the honoring of God. Indeed, they worship both God and the saints indiscriminately, except that, when they are pressed, they wriggle out with the excuse that they keep unimpaired for God what is due him because they leave latria to him. But since the thing itself, not the word, is in question, who can permit them to make light of this most important of all matters?
But — to pass over this also — their distinction in the end boils down to this: they render honor [cultus] to God alone, but undergo servitude [servitium] for the others. For latreia, among the Greeks means the same thing as cultus among the Latins; douleia properly signifies servitus; and yet in Scripture this distinction is sometimes blurred. But suppose we concede it to be unvarying. Then we must inquire what both words mean: douleia is servitude; latreia, honor. Now no one doubts that it is greater to be enslaved than to honor. For it would very often be hard for you to be enslaved to one whom you were not unwilling to honor. Thus it would be unequal dealing to assign to the saints what is greater and leave to God what is lesser. Yet many of the old writers used this distinction. What, then, if all perceive that it is not only inept but entirely worthless?
Source: Calvin's Institutes, 1:12:2 (Battles Translation)
Alternate translation: (Beveridge Translation)
"The distinction of what is called dulia and latria was invented for the very purpose of permitting divine honors to be paid to angels and dead men with apparent impunity. For it is plain that the worship which Papists pay to saints differs in no respect from the worship of God: for this worship is paid without distinction; only when they are pressed they have recourse to the evasion, that what belongs to God is kept unimpaired, because they leave him latria. But since the question relates not to the word, but the thing, how can they be allowed to sport at will with a matter of the highest moment? But not to insist on this, the utmost they will obtain by their distinction is, that they give worship to God, and service to the others. For "latreia" in Greek has the same meaning as worship in Latin; whereas "douleia" properly means service, though the words are sometimes used in Scripture indiscriminately. But granting that the distinction is invariably preserved, the thing to be inquired into is the meaning of each. "Douleia" unquestionably means service, and "latreia" worship. But no man doubts that to serve is something higher than to worship. For it were often a hard thing to serve him whom you would not refuse to reverence. It is, therefore, an unjust division to assign the greater to the saints and leave the less to God. But several of the ancient fathers observed this distinction. What if they did, when all men see that it is not only improper, but utterly frivolous?