There were two different gospels preached during the Acts period.
One declared that Jesus is the Christ, the Son of God, and those who believed it were saved the moment they believed (Jn.20:31; 1 Jn.5:1-5).
The other gospel is the gospel of grace which is centered on the fact that Christ died for our sins and belief in that gospel saves all those who believe it (1 Cor.15:1-4).