The "church of the firstborn" is the Jewish "Sect of the Nazarenes", a strictly Jewish sect.
Mosaic Law was still practiced by the Jewish believers in Acts.
Rejection of Steven's message = Steven stoned = Paul (apostle to Gentiles) saved.
Just because they use the Greek ekklesia does not mean the Body of Christ.
2 Tim 2:15 "rightly dividing"
The "middle wall of partition" between those saved under the Gospel of the (imminent) Kingdom
(ie: "grafted in" to Israel)
and those saved by the Gospel of Grace ("one new man") was accomplished in Acts 28:28 when
Paul is in prison (see Ephesians).