John Calvin Commentary Genesis 14:20

"and blessed be God Most High, who hath delivered thine enemies into thy hand. And he gave him a tenth of all." — Genesis 14:20 (ASV)

And he gave him tithes of all. There are those who understand that the tithes were given to Abram; but the Apostle speaks otherwise, declaring that Levi had paid tithes in the loins of Abram (Hebrews 7:9), when Abram offered tithes to a more excellent Priest. And truly, what the previously mentioned expositors mean would be most absurd, because if Melchizedek was the priest of God, it was his place to receive tithes rather than to give them.

Nor can it be doubted that Abram offered the gift to God, in the person of Melchizedek, in order that, by such first-fruits, he might dedicate all his possessions to God. Abram therefore voluntarily gave tithes to Melchizedek to honor his priesthood. Moreover, since it appears that this was not done wrongfully nor rashly, the Apostle properly infers that, in this figure, the Levitical priesthood is subordinate to the priesthood of Christ.

For other reasons, God afterwards commanded tithes to be given to Levi under the Law; but, in the age of Abram, they were only a holy offering, given as a pledge and proof of gratitude. It is, however, uncertain whether he offered the tithe of the spoils or of the goods which he possessed at home.

But, since it is improbable that he should have been liberal with other persons’ goods, and should have given the very tenth part of the prey, of which he had resolved not to touch even a thread, I rather conjecture that these tithes were taken out of his own property.

I do not, however, admit that they were paid annually, as some imagine; rather, in my judgment, he dedicated this present to Melchizedek once, for the purpose of acknowledging him as the high priest of God. Nor could he, at that time (as we say), hand it over; but there was a solemn stipulation, the effect of which followed shortly after.