Romans 4:3 — For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.
Paul's answer to his question in verse one isn't from logic, but from Scripture. And he believed in the LORD; and He counted it to him for righteousness (Genesis 15:6).
Count (reckon, impute) = lay to one's charge, charge to one's account
The man who trusts Christ becomes, in Him, all that God requires a man to be.
Abraham believed God that his seed should be a nation. He didn't know about Christ — who actually saved him). But his belief in God and what God said was reckoned as righteousness until the cross.