On rock

Coins also provide evidence of the effort to deify Livia. In some cases she is depicted as Demeter/Ceres, the goddess of grain. Philip issued coins in honor of Livia in the year 30CE, some stamped with the inscription karpophoros "fruitbearer", a clear allusion to Demeter/Ceres. Philip's coins leave no doubt as to his commitment to the Roman imperial cult and his specific loyalty to Livia herself. The linkage between the honoring of Livia and the refounding of Bethsaida is seen in coins issued in 30 CE that read "by Philip the tetrarch, founder" and "Julia Sebaste". "Founder" (ktistes) refers to the refounding of Bethsaida. 

The heights of Bethsaida rest on a rocky ridge of volcanic basalt. The earliest phase of the city reaches back to the Bronze Age. The site was chosen because it is the highest ground along the north shore of the Sea of Galilee, the ridge provides plenty of rock for building and defensive purposes, there is ready access to fresh water (such as the Jordan and Meshosim Rivers, as well as the Sea of Galilee) and there is also abundant arable land surrounding the ridge. Indeed, the original name of Bethsaida may have been Zer ("rocks") [as seen in the old geographical list in Josh 19:35, which lists "the fortified cities, Ziddim, Zer, Hammath, Rakkath, Kinneret", cities that encircle the Sea of Galilee]. It was on this "rock" that Philip the tetrarch refounded Bethsaida and began building the new city of Julias in honor of the Roman matriarch, now divinized in death (by order of the will of Augustus).

I wonder if Philip's recent announcement to rename Simon's hometown and to build a temple to honor Livia atop its rocky precipice was the actual incident that prompted Jesus to call Simon rock and promise to build his church on solid bedrock. Philip the tetrarch, the faithless son of Herod the Great, has betrayed the God of Israel and his people by embracing and promoting the rank paganism of the Romans, who worship humans and regard them as gods (see, for example, the sentiment of Paul the former Pharisee in Rom 1:22-23,25). Philip has announced that he will rename Bethsaida, the hometown of Peter and other disciples, to honor Livia, the widow of Augustus, whose divine name became Julia Augusta. In her honor and probably to support the cult of Livia and the drive to deify her Philip the tetrarch intends to build a temple on the rocky prominence of Bethsaida. In reaction to this plan Jesus promises to build his church, a community of confessing followers like Simon Peter, against which the very gates of Hades  will not prevail. That is, the political and demoniacal powers (and not simply "powers of death", as in the RSV's translation) that back Philip will not overcome the community that Jesus foresees growing up around the confession of Peter, namely, that Jesus is the Anointed One, the Son of God. C. Evans "'On This Rock I Will Build My Church' (Matthew 16:18): Was the Promise to Peter a Response to Tetrarch Philip's Proclamation?", Aaron White, David Wenham, & Craig A. Evans, eds. The Earliest Perceptions of Jesus in Context: Essays in Honor of John Nolland (Bloomsbury 2018), chap. 2.