To say faith alone is the sin of idolotry.
Was Abraham made righteous, or was his faith credited to him as righteousness? Abraham was not made righteous, was not born anew, was not sealed in Christ with the Holy Spirit forever. None of this has been rebutted, only evaded.
Did the OT Saints have to wait to be made perfect? Yes, see Hebrews 11:39-40. Thus they were not made perfect before Christ died, did not enter heaven before Christ died (John 3:13) and the new birth therefore did not occur before Christ died.
As far as the mistaken view the fall made men unable to respond to the gospel, that is simply unbiblical. Take a look at Matthew 23:13 where unregenerate men were "entering heaven." How could they be entering heaven if they were not seeking God and His righteousness? Want more evidence, consider Ephesians 2:8 where we are saved by grace through faith. Thus we receive the grace of salvation through faith. Picture a room called salvation and the room is filled with grace. Now go through the door of faith. Thus your faith preceded your salvation or the grace of salvation.
Here are some scriptures to help you understand, no Grace from God (unmerited favor) no faith, faith is a work an action done by someone who has been regenerated by Grace, to say faith alone is the sin of idolatry.
Rom 4:1 What shall we say then that Abraham our father, as pertaining to the flesh, hath found?
Rom 4:2 For if Abraham were justified by works, he hath whereof to glory; but not before God.
Rom 4:3 For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.
Rom 4:4 Now to him that worketh is the reward not reckoned of grace, but of debt.
Rom 4:5 But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness.
Rom 4:6 Even as David also describeth the blessedness of the man, unto whom God imputeth righteousness without works,
Rom 4:7 Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered.
Rom 4:8 Blessed is the man to whom the Lord will not impute sin.
Rom 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.
Rom 4:10 How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision.
Rom 4:11 And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also:
Rom 4:12 And the father of circumcision to them who are not of the circumcision only, but who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised.
Rom 4:13 For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith.
Rom 4:14 For if they which are of the law be heirs, faith is made void, and the promise made of none effect:
Rom 4:15 Because the law worketh wrath: for where no law is, there is no transgression.