Genesis 14:20

People practiced tithing as an act of worship commonly in the ancient Near East at this time (cf. Genesis 28:22). [Note: See Keil and Delitzsch, 1:207.] It was also a common tax. This is still true in some modern countries. For example, in England part of every person’s taxes goes to maintain the Church of England. Some residents regard this part of their tax as their contribution to the church or their tithe. However since Melchizedek gave Abram a priestly blessing, it is likely that Abram reciprocated by giving Melchizedek a gift with priestly connotations. [Note: Wenham, Genesis 1-15, p. 316.] "All" probably refers to all that he took in the battle rather than all that was in Abram’s possession (cf. Genesis 14:23-24; Hebrews 7:4).