God has made many promises, and many of them have been fulfilled. Steven reminds the Pharisees of God's promise to Abraham, His promise to Moses, and now Steven is trying to catch them up on the promise of the coming Messiah. At this point the Messiah has already come and when he came Jesus promised to send the Holy Spirit - we see God fulfill that promise in Acts 2. Ultimately, Acts chapter 7 is a reminder that God keeps his promises and we can always trust him.
References:
God promises to send another prophet like Moses: Deuteronomy 18:15-18, John 1:21, John 6:14, Acts 3:22
Prophesies about Jesus birth:
Isaiah 7:10, it was fulfilled in Matthew 1:18-23,
Prophesies about the Messiah being crucified:
Psalms 22:16-18, it was fulfilled in Matthew 27:35
Jesus promises the Holy Spirit:
John 14:26, John 16
We see glimpses of the Holy Spirit come:
Luke 3:21, Luke 1:41-42, then in Acts 2 we see the Holy Spirit rest on the church.
