"Abraham believed" - a closer look! "Believed" is in the Aorist tense and this dynamic grammatical tool should not be overlooked. It would be easy as English speakers to interpret this verb as a singular "event" or as some scholars say "simple past tense". However, the beauty of the Aorist tense is seen in its intentional use as a "summary statement." The idea is looking back over a span of time and summarizing the span as a whole.
Moses looking back upon Abraham's life and Paul drawing from Moses, "summarized" Abraham's life using the Aorist tense. He summed up Abraham's life with "Abraham believed."
How can we know that Paul intended the reader to see this as a "summary" of Abraham's walk rather than a singular event? We can see this by the description of Abraham given in Romans chapter 4, especially in verses 18-22.
These verses describe a span of years rather than a single event. Abraham hoped against hope for many years, and it was over a long period of time he was not staggered by unbelief. He became strengthened by faith through the process of having to face his age and the deadness of Sarah's womb. Many times, over and over again, he chose to believe what God had promised - until he came to the full assurance of faith and became fully persuaded by grasping the truth that God had power to do what He had promised. This is why Abraham is the premier example of the righteousness of God FROM faith TO faith.
Paul even concluded that the statement "and it was reckoned unto him unto righteousness" was not written solely for him but for ALL who WALK IN THE FOOTSTEPS OF FAITH, which Abraham had while he was still uncircumcised.
Those who live by faith in Christ, walking in the footsteps of faith as they abide in Christ, from faith to faith with CHRIST abiding in their heart, will have righteousness reckoned unto them just like our father Abraham.
This is what the Son of Man will be looking for when He returns, people living by faith, walking in the faith, servants found faithfully serving. He will not be looking for what someone believed at one time, but for those who are believing, still walking by faith and continuing in the grace that is accessed by faith.
