All that believed in God's promise of a savior were justified by faith alone. Faith and faith alone is the justifier, Romans 3:26.
Problem is, that is not what Romans 4 asserts about what Abraham had believed. Not in Romans 4.
Would that the evidence against your assertion ends with the following.
Genesis 15:2 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? 15:3 And Abram said, Behold, to me thou hast given no seed: and, lo, one born in my house is mine heir. 15:4 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. 15:5 And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be. 15:6 And he believed in the LORD; and he counted it to him for righteousness.
But the evidence does not end there. You are simply wrong on this one.
Romans 4:3 For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.
When was that?
Romans 4:9 Cometh this blessedness then upon the circumcision only, or upon the uncircumcision also? for we say that faith was reckoned to Abraham for righteousness. 4:10 How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision.
Abraham was still in uncircumcision when his above belief of what God had told him was counted to him for righteousness.
And Isaac was not even conceived, let alone a child Abraham would later have obeyingly sacrificed, but for God's intervention, Gen. 22.
Again, Romans points back to Genesis 15...
Romans 4:16 Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all, 4:17 (As it is written, I have made thee a father of many nations,) before him whom he believed, even God, who quickeneth the dead, and calleth those things which be not as though they were. 4:18
Who against hope believed in hope, that he might become the father of many nations, according to that which was spoken, So shall thy seed be.
Again, what had been that which had been spoken unto him, then?
Genesis 15:3 And Abram said, Behold, to me thou hast given no seed: and, lo, one born in my house is mine heir. 15:4 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. 15:5 And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be. 15:6 And he believed in the LORD; and he counted it to him for righteousness.
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: 4:20 He staggered not at the promise of God through unbelief; but was strong in faith, giving glory to God; 4:21 And being fully persuaded that, what he had promised, he was able also to perform. 4:22 And therefore it was imputed to him for righteousness.
"The mouth of" these "two or three witnesses" is what you are going against.
2 Corinthians 13:1 This is the third time I am coming to you. In the mouth of two or three witnesses shall every word be established.
This also being my third attempt, I'll leave you at these witnesses against you.
Romans 5:6-8, in each - our stead.