Charles Ellicott 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)

He gave him tithes. —Abram thus consecrated the war by a thank-offering to God, Who had given him the victory. But he also, by paying tithes, acknowledged the priesthood of Melchizedek, and that the God Whom he served was the true God. See Hebrews 7:4-11.