Since verse 12 spoke about "But of those through faith and long-suffering, inherit the promises."
Verses 13-15 goes onto explain another promise that God made. He uses the example of the promise made to Abraham. God kept the promise because Abraham patiently endured thru faith so Abraham obtained the promise offered by God.
Verses 13-15
13. For when God made promise to Abraham, because he could swear by no greater, he sware by himself,
14. Saying, Surely blessing I will bless thee, and multiplying I will multiply thee.
15. And so, after he had patiently endured, he obtained the promise.