The response to Peter's sermon is dramatic. About three thousand Jews accept Peter's message and are baptized. We can assume both that they repented of their past sins and also received the Holy Spirit (Acts 2:38). If the Feast of Pentecost was a celebration of the harvest, then these are the "first fruits" of the age of the Spirit and the new covenant.
Presumably, every one of these three thousand people was a Jew. That is to say, they were not changing religions. They were not leaving Judaism to Christianity. They were becoming better Jews. They were believing in the Jewish Messiah. They were repenting of their failure to be faithful to God's covenant with Israel. And they were receiving the promised Spirit of the Scriptures of Israel.
This was as much the renewal of Israel as the birth of the church--at least it should have been. It would not always be this way. In the early days, the number increased first to around five thousand Jews in Acts 4:4, and then 5:14 speaks of a "multitude" constantly being added. But we know from Paul's letter to the Romans that most Jews would not end up believing. In Romans 9-11, Paul wrestles with the paradox that although the good news was first for Israel, most of Israel did not believe it at that time.
We should thus be careful not to assume that church growth--especially massive church growth--is always going to be the norm. We can sometimes get the impression that if a church is doing all the right things, it will not only grow in numbers but will grow astronomically. But we should remember that the beginning of the church was somewhat of a unique event. Growth did not continue on this magnitude for long in Jerusalem.
The Jewish historian Josephus barely even mentions Jesus in his history of this period.  That is to say, Christianity was not a major feature of Israel--at least not to him--in the late first century. Sometimes the church grows. Sometimes it grows fantastically. But sometimes Jesus cannot perform miracles because a location lacks faith (cf. Mark 6:5-6). He does not force people to believe. Sometimes the gate is small and the road is narrow, and few find it (Matt. 7:14). Because God gives humanity a choice, nothing anyone does can guarantee that others will believe...
 Josephus' mention of Jesus in Antiquities 18.5.2 has been tampered with, leading some to argue that Josephus never mentioned Jesus at all. However, it is more likely that Josephus briefly mentioned Jesus and that some later Christian copyists "enhanced" his description.