5 There is much to tell of faith, and the whole day would not be time sufficient for us to describe it fully. At present let us be content with Abraham only, as one of the examples from the Old Testament, seeing that we have been made his sons through faith. He was justified not only by works, but also by faith: for though he did many things well, yet he was never called the friend of God, except when he believed. Moreover, his every work was performed in faith. Through faith he left his parents; left country, and place, and home through faith. In like manner, therefore, as he was justified be thou justified also.
In his body he was already dead in regard to offspring, and Sarah his wife was now old, and there was no hope left of having children. God promises the old man a child, and Abraham without being weakened in faith, though he considered his own body now as good as dead, heeded not the weakness of his body, but the power of Him who promised, because he counted Him faithful who had promised, and so beyond all expectation gained the child from bodies as it were already dead.
And when, after he had gained his son, he was commanded to offer him up, although he had heard the word, In Isaac shall your seed be called, he proceeded to offer up his son, his only son, to God, believing that God is able to raise up even from the dead. And having bound his son, and laid him on the wood, he did in purpose offer him, but by the goodness of God in delivering to him a lamb instead of his child, he received his son alive. Being faithful in these things, he was sealed for righteousness, and received circumcision as a seal of the faith which he had while he was in uncircumcision, having received a promise that he should be the father of many nations.
Source: Catechetical Lectures (New Advent)