Just as always only God knows for sure. We do know that many Jews responded to Christs message and were called disciples until His sermon recorded in Matthew after which many of them left. Many were added at the sermon at Pentecost and we can only assume that because we are told that that they indeed were saved. It does say they received the Holy Spirit.
Keep in mind. That many became believers prior to Pauls revelation of the mystery doesnt detract from the fact that a mystery still hadnt been revealed. Many today are saved not realizing the free gift they received. They may even have thought that it was they who did something. That isnt the point.
The main point of understanding the mystery is to understand that salvation is a true gift not dependent on mans actions. Once people understand that there is a deeper understanding of how God will deal with people after the rapture. Once again it will be actions that count. That is clear from the command to not take the mark of the beast. Salvation is no longer a free gift but requires people to take action and persevere.
Understanding the mystery that salvation is a free gift and entirely the work of the Holy Spirit within us we are relieved of the guilt that Satan tries to plant in our minds. He can no longer plant that doubt because I can say that the Holy Spirit within me is my guarantee or seal that I belong to God no matter what I do. Not to say I want to do wrong because the Holy Spirit is now my conscience if you will but the guilt is gone because its not something I have to other then have faith in the shed blood if Jesus and that I am secure in Christ. He chose me, I didnt choose him.
Many people are saved who never understood the mystery. For one, I dont think they had the peace of mind we do in the security of salvation and two, they never understood the prophesy of the coming tribulation.
I pray that helps explain. The Old Testament saints never had that assurance and neither will those during the tribulation.
As far as Isaiah 53, WHO was he referring to when he talked of "all we like sheep", "for the transgression of MY PEOPLE was he stricken", etc. Verse 1 tells you: ZION. Jerusalem, ISRAEL. Yes, the gentiles will also be benefactors of Isa. but it will be THROUGH ISRAEL, God's kingdom of priests and a blessing to all nations, during the Millenial reign of Christ that this will happen. RIght now is different, as we are seeing. The grace of God, the body of Christ, etc. til the fullness of the Gentiles be brought in.
I have met people like that. Those who were brought up in certain religious denominations who are totally sold out to Christ and yet I am convinced, didn't really understand what they did.