When all they had was the Law of Moses, obedience to it was acceptable to God.
Act 10:1-2 There was a certain man in Caesarea called Cornelius, a centurion of the band called the Italian band, (2) A devout man, and one that feared God with all his house, which gave much alms to the people, and prayed to God alway.
Act 10:34-35 Then Peter opened his mouth, and said, Of a truth I perceive that God is no respecter of persons: (35) But in every nation he that fears him, and works righteousness, is accepted with him.
But here's where faith gets the better of the Law...
Galatians 3:22-24 The Purpose of the Law
22 But the Scripture pronounces all things confined by sin, so that by faith in Jesus Christ the promise might be given to those who believe.
23 Before this faith came, we were held in custody under the Law, locked up until faith should be revealed.
24 So the law became our guardian to lead us to Christ, that we might be justified by faith.…
Berean Study Bible ·
And even if one is blameless as far as the righteousness of the Law is concerned the result is worthless compared to the righteousness which is through the faith of Christ and is of God by faith.
Php 3:6-11 Concerning zeal, persecuting the ekklesia; touching the righteousness which is in the law, blameless. (7) But what things were gain to me, those I counted loss for Christ. (8) Yea doubtless, and I count all things but loss for the excellency of the knowledge of Christ Jesus my Lord: for whom I have suffered the loss of all things, and do count them but dung, that I may win Christ, (9) And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith: (10) That I may know him, and the power of his resurrection, and the fellowship of his sufferings, being made conformable unto his death; (11) If by any means I might attain unto the resurrection of the dead.
Yea, pretty well said, although we may see some of this a little differently--I'm not sure. When Paul says "faith," again, he is speaking in "shorthand," expressing "faith" in its more elaborate form--"faith in Christ," or "faith for salvation." The righteousness of the Law operated by faith in the OT, but it was not yet "faith in Christ" because faith had not yet come. When Christ came, "faith in Christ" came, and previous faith under the Law was fulfilled as "faith in Christ."
So saints were accepted by their faith under the Law, and their righteousness was real. But they were not permanently justified until the "faith in Christ" came, to complete faith in an eternal object--Christian atonement. Faith under the Law was a faith in temporary atonement, and had hope that God would find a means of eternal redemption, which of course happened when Christ came.
In other words, faith in the OT trusted in temporary sacrifices like animal sacrifices for a temporary means of atonement, and as a temporary means of maintaining a relationship with a holy God. Obeying God's word made the faith of the saints acceptable before God until God perfected their faith in an eternal atonement, Christ.
OT faith did qualify for NT faith, because the object of faith is not faith itself, but rather, the object of that faith, which is God's word. And God's word has resolved temporary atonement in the eternal atonement of Christ. All those who truly obeyed God under the Law in faith qualify for the eternal atonement of Christ. Those who truly obeyed the Law from the heart committed their whole heart and nature to God, and thus were very similar to today's born again Christian.