In the loins of his father (ἐν τῇ ὀσφύΐ τοῦ πατρὸς)
His own father; not of Abraham.
When Melchisedec met him
In the person of Abraham. The whole Jewish law, its ordinances and priesthood, are regarded as potentially in Abraham. When Abraham paid tithes, Levi paid tithes. When Abraham was blessed, Israel was blessed. It is a kind of reasoning which would appeal to Hebrews, who so strongly emphasized the solidarity of their race. Comp. Rom 9:4, Rom 9:5.