The writer offered Abraham as an encouraging and supreme example of one who continued strong in faith and patience. [source][source][source]
"There is in Hebrews a sustained interest in Abraham ( Hebrews 2:16; Hebrews 6:13-15; Hebrews 7:4-5; Hebrews 11:8-19). The appeal to Abraham as a prototype of faithful endurance in Hebrews 6:13-15 gives specific content to the exhortation in Hebrews 6:12." [1][source]
The promise to which the writer referred was the one God gave Abraham after he had obeyed God by offering up Isaac (cf. James 2:21). Abraham trusted God to fulfill His former promise regarding his descendants by raising Isaac from the dead ( Genesis 22:16-17). The writer was calling his readers to do what God called Abraham to do when He instructed him to go to Mt. Moriah. They too needed to continue to trust and obey, as they had done in the past, even though it looked as though perseverance would result in tragedy. Having patiently waited and remained steadfast in the face of trying circumstances, Abraham qualified to receive everything God wanted to give him (cf. Colossians 1:11; Hebrews 12:1-3; Hebrews 12:7; James 5:11). [source][source][source]