Romans 4:19 And being not weak in faith, he considered not his own body now dead, when he was about an hundred years old, neither yet the deadness of Sarah’s womb: 20 He staggered not at the promise of God through unbelief; but was strong in faith, giving glory to God; 21 And being fully persuaded that, what he had promised, he was able also to perform. 22 And therefore it was imputed to him for righteousness.
Abraham was not weak in faith. Where did Abraham’s strength originate? It was from God. Where did his life come from? It was from Christ, the Son of God. Abraham was strong in Christ. Look in Genesis.
Genesis 15:1 After these things the word of the Lord came unto Abram in a vision, saying, Fear not, Abram: I am thy shield, and thy exceeding great reward. 2 And Abram said, Lord God, what wilt thou give me, seeing I go childless, and the steward of my house is this Eliezer of Damascus? 3 And Abram said, Behold, to me thou hast given no seed: and, lo, one born in my house is mine heir. 4 And, behold, the word of the Lord came unto him, saying, This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir.
The Lord came to Abraham saying fear not. Abraham had no heir and offered God the solution. God said no, that is not My plan.
Genesis 16:4 And he went in unto Hagar, and she conceived: and when she saw that she had conceived, her mistress was despised in her eyes.
Abraham would have a son by the handmade of his wife Sarai. It was their attempt to fulfill the promise of God of an heir. Did Abraham trust in God fully in both instances? I would say no as we can see Abraham tried to offer his own solution. If God required perfect faith, if He required us to follow and obey perfectly none would have hope. His strength was God Himself. His strength was that God was able to carry out the promise of salvation given to Abraham all those years ago. God, because He was able to do this imputed faith, imputed that finished work of Christ, the merit of His work to Abraham for righteousness. When was this imputed? When the propitiation that was the Son of God came and shed His blood.