For Abraham, the relationship between his faith, and the basis of his justification was upon the promised seed, who was Jesus Christ (Gal. 3:16). Abraham is the father of many nations in the sense that through his offspring (Jesus), all the nations have been blessed as Christ has redeemed men from every tribe, tongue, and nation (Rev. 5:9-10). In His sovereign grace, God covenanted with Abraham and made this promise which was received by Abraham in faith (complete trust and reliance). Abraham believed God about the promised seed that was to come who would bring about the blessing (which is justification before God by faith), and it was accounted to him for righteousness.
In just the same way, our trust is in the promised seed (offspring) who has come. In grace, God sent forth His Son who willingly was obedient unto death upon the cross. By faith, we believe God