Question

What was GOD's plan of salvation before the Cross and the Day of Pentecost and then the plan of salvation after the Apostles met with 120 on the Day of Pentecost?


That’s a very big question.
If we look at the OT which includes the Gospels and the first part of the Acts we see that the concept of salvation is bound up in the promises of God to Abraham in which God promises blessing to Abraham and to his seed (Israel and Christ). This blessing was going to come to them as a nation and through them to the whole world. Salvation was a national thing rather than a individual thing. During this time those who lived righteously because they believed in God were promised resurrection to enter the coming Messianic Kingdom. Even after Christ's resurrection the disciples were still asking, are you at this time going to restore the Kingdom to Israel?
After the day of Pentecost God began to do something completely new. God began to call Gentiles and Jews to personal faith in Christ based upon not what they have done or the righteous lives they have lived, but on the basis of grace to the unworthy. In the present day God is calling all men including Jews to come to him and when they do they become members of the church – the body of Christ. When the church is caught up to heaven God will revert to the previous arrangement in which he will bless men who believe in him and live righteously under the law, When the Kingdom dawns on the world all the righteous of all ages will be raised from the dead and will enter into the blessings of the New Covenant. And so on that day the righteous will catch up as it were to the blessings which the church has enjoyed for 2000 years.

Steve