Were early Christians were saying “Jesus resurrected”?
Regarldess of how one defines "resurrection" (Greek: anastasis), where early Christias—i.e. believers in c. AD 30—publically maintaining that Jesus really died and afterwards was eschatologically raised from the dead (where by eschatological resurrection we mean to refer to the basic kind of resurrection Jews anticipated for all humans at the end of history).
1st Christians suddenly deemed Sunday special
The first Christians felt something very theologically significant happened on a Sunday (“The First day of the week”).
This is relevant because absent some significant inciting event (like belief that Christianity's central event—Jesus' resurrection—took place on that day), nothing would've lead them to judge it so theologically significant.
E.g. Peter was saying “Jesus resurrected!”
The apostle Peter was publicly proclaiming that “Jesus resurrected!”
A full page will analyze these 7 arguments:
This is relevant because Peter was an early Christian, and moreover Christians at large followed him in his teachings/belief regarding Jesus (including that Jesus rose and appeared to him).