"Against all hope, Abraham in hope believed and so became the father of many nations." (Romans 4:18)
Paul turns to Abraham to demonstrate that justification by faith is not a novelty but the original pattern of God's dealing with humanity. What did Abraham discover? If Abraham was justified by works he had something to boast about, but not before God. What does the Scripture say? Abraham believed God, and it was credited to him as righteousness. He quotes Genesis 15:6, the moment of the covenant promise. The critical point Paul makes is temporal: Abraham was credited with righteousness while he was still uncircumcised. Circumcision came later, as a sign and seal of the righteousness he had already received by faith. Therefore Abraham is the father of all who believe, both circumcised and uncircumcised. The covenant was made with the believing Abraham, not the circumcised Abraham.
The promise to Abraham and his offspring that he would be heir of the world came not through the Law but through the righteousness of faith. If those who depend on the Law are heirs, faith means nothing and the promise is worthless. The Law brings wrath, but where there is no Law there is no transgression. Therefore the promise comes by faith, so that it may be by grace and may be guaranteed to all Abraham's offspring, not only those under the Law but also those who share the faith of Abraham, who is the father of us all.
Against all hope, Abraham in hope believed and so became the father of many nations. He faced the fact that his body was as good as dead, and that Sarah's womb was also dead. Yet he did not waver through unbelief regarding the promise of God, but was strengthened in his faith and gave glory to God, being fully persuaded that God had power to do what he had promised. This was credited to him as righteousness. The Catechism identifies Abraham's faith as the paradigm of all theological faith: not the evidence of things seen but the confident trust in the faithfulness of the one who has promised (CCC 146). Against all human hope, in hope believing.
Brothers and sisters, Abraham did not weaken in faith when he faced the fact of his and Sarah's age. He faced it fully, and believed anyway. This is mature faith: not the avoidance of hard facts but the choice to trust God's promise in the full knowledge of contradicting circumstances. What promise of God are you facing that seems contradicted by the facts? Face the facts, as Abraham did. Then believe God, as Abraham did.
Lord God, you called Abraham to believe against all hope and he believed and it was credited as righteousness. Give us Abraham's faith: fully aware of the obstacles, fully trusting in your promise, giving you glory because you have the power to do what you have promised. Through Christ our Lord. Amen.