Abraham's 'multitude' wasn't just about physical descendants. It pointed to something much bigger, encompassing people of faith across all nations.
While Abraham would indeed father many literal nations through his descendants (Ishmael, Isaac, and the sons of Keturah), the promise extended far beyond the physical. The commentators highlight that 'father of a multitude of nations' also refers to Abraham being the spiritual ancestor of all who believe. This means people from every background, tribe, and tongue would, through faith in God's promises (which found their ultimate fulfillment in Jesus Christ), be considered Abraham's spiritual offspring. His legacy wasn't confined to bloodlines but expanded to include a global family united by faith.