After dealing with the first period of revealed history from the creation of the world to the flood our writer moves on to the second period. Abraham and his immediate offspring form the second link in the chain of examples of God-pleasing faith in our writer's list. This makes sense, not only chronologically, but considering our writer's audience. These converted Hebrew believers were facing the hostility and push-back from their Jewish leaders and family members. In their decision to follow Christ they were - like Abraham - leaving much of their former world behind and this was proving to be an unpopular decision.
So our writer very wisely points them to their spiritual father. Abraham is the first person said to have "pleased God" after the flood. And the first thing our writer mentions is the call of Abraham. There were features of Abraham's call - something in his example - these Jewish believers needed to emulate as he ventured out in stubborn faith, leaving his former world behind.