And he blessed him and said,
“Blessed be Abram by God Most High,
Possessor of heaven and earth;
and blessed be God Most High,
who has delivered your enemies into your hand!”
And Abram gave him a tenth of everything.
Abram had just returned from defeating the armies of the four kings, rescuing his nephew Lot, and reclaiming his possessions, and he’s met by an enigmatic priest of God Most High named Melchizedek. The priest attributes Abram’s victory to God—possessor of heaven and earth—and he blesses Abram.
In gratitude to God’s authority and blessing, Abram gives Melchizedek a tenth of his possessions. He doesn’t do it to invoke God’s blessing; he does it in response to God’s blessing.