The earliest Christians generally did not use the cross as a symbols of their religion. (It’s major emergence as a symbol of veneration came with the discovery of the True Cross by Helena, mother of Constantine, in a cistern underneath the Holy Sepulcher in Jerusalem.) One of the earliest Christian symbols was, in fact, the fish, in Greek “ichthus” (ΙΧΘΥΣ), which formed an acronym for “Jesus Christ, God’s Son, Savior”
Ίησοῦς = Iēsous = Jesus
Χριστός = Christos = Christ
Θεοῦ = Theou = “of God”
Υἱός = (H)uios = Son
Σωτήρ = Sōtēr = Savior
The original early fourth century mosaic floor of Constantine’s Basilica of the Nativity in Bethlehem has one of the earliest representations of this symbol.
Here is a detail of the central box of the mosaic.
Although partially damaged, you can see block forms of the Greek letters forming ichthus/ΙΧΘΥC/fish, with the final sigma (the “S” sound, looking like our “C,” but also written as an Σ) underneath the first four letters.
As found in this graffiti from Ephesus: