What did Abraham do? The word "accounting" (KJV) or "concluding" (NKJV) tells us a great deal. In Greek, it is an accounting term. An accountant adds up figures. He puts them all in a ledger. He writes down all of the receipts in one column and all the expenditures in another. He adds each column to get their totals. Then, he has an accounting of his or his company's financial health. The numbers are evidence of his own or the company's state.
Abraham did this too, only instead of adding numbers, he added up evidence. His evidence came from the words that God told him: "You shall have a son." It took 25 years, but he did indeed have a son.
A number of years later, God said to him, "Abraham, I want you to go out and sacrifice your son." Abraham could have said, "Uh oh, there's evidence that I didn't count on." But, instead, Abraham left for Mount Moriah early the next morning. What evidence did he have to motivate him to do in faith what God commanded him to do? The Word of God. God had earlier told Abraham that the promise would come through his son, Isaac—not through Ishmael, not through any future son that he might have, but through Isaac, the promised son.
What did Abraham do with this evidence? He knew that there could be only two possible outcomes. If God required Abraham to put Isaac to death, then He would resurrect him, or if God was not going to require Abraham to kill Isaac, then God would give him a substitute sacrifice. Either way, Isaac would live. Abraham added up the evidence, and it produced the motivation to do what he had to do in faith.