Bob, I reckon what they believed, if they were true believers, was what Peter preached at Pentecost and what the Lord continued to reveal during whatever life they had left before the conversion and ministry of Paul.
Acts open with this:
Act 1:1 The first account, indeed, I (Luke) make, O Theophilus, concerning all which Jesus begins both to do and to teach,
Act 1:2 until the day on which He was taken up; through holy spirit directing the apostles whom He chooses,
Act 1:3 to whom also He presents Himself alive after His suffering, with many tokens, during forty days, being visualized to them and telling them that which concerns the kingdom of God."
He just a bit later adds this from what the Lord said after His resurrection:
Act 1:6 Those, indeed, then, who are coming together, asked Him, saying, "Lord, art Thou at this time restoring the kingdom to Israel?
Act 1:7 Yet He said to them, "Not yours is it to know times or eras which the Father placed in His own jurisdiction."
If they were "expecting" a Messianic kingdom in their lifetime, they had no real right to do so.
Acts 2 is the sermon Peter preached that day. You can see for yourself what he said to and about them and what they were required to do to "OBEY" the gospel. No need to post the whole chapter here, but first they were required to REPENT and be Baptized.
Some 3000 did. And afterwards:
Act 2:42 And they were continuing steadfastly in the doctrine of the apostles, and in fellowship, and in the breaking of bread, and in prayers.
Act 2:43 And fear came to every soul, and many wonders and miraculous signs took place through the apostles.
Act 2:44 And all the believers were together and had all things common.
Act 2:45 And they sold possessions and goods and distributed them to all, according as anyone had need.
Act 2:46 And continuing steadfastly with one mind day by day in the temple, and breaking bread from house to house, they shared food in gladness and simplicity of heart,
Act 2:47 praising God, and having favor with all the people. And the Lord added to the assembly, the ones being saved from day to day.
If they were believers, that's what they believed and did--collectively, the Doctrine of the Apostles. I can't fathom a theology that suggests that the Apostles' doctrine was different from that which Jesus preached and modeled.
Of course, that's just the simplified version. A number of things still had to be revealed.