In Genesis 14:18-20 (cached), after Abram's victory over Chedorlaomer, Melchizedek King of Salem comes to serve as a priest in sacrificing bread and wine and blessing Abram, who thereupon gives him a tenth of everything. In Christian typology, this made Melchizedek a type of Christ (c.f. Hebrews 5:5-7: "Thou art a priest for ever, according to the order of Melchisedech.") and of the priesthood that Christ would establish.
At left, from an 11th century portable altar