Without an imperial church granting growing state backing for orthodoxy I think Christianity would diverge and cluster into two or three early denominations around Rome, Antioch, and Alexandria and kind of obviously but still interestingly not Constantinople. The power to enforce orthodoxy would not have the same kind of state support, so it would have less bite, within these hypothetical churches as well as over the whole community too. They wouldn't divide indefinitely like amoebas and exeunt into the religious background though, not with a POD in the 4th century.
Christian religions might agree on Jesus as a resurrected saviour and pretty much nothing else due to diversion over time, or they might just reflect the greater diversity of the contemporary church and disputes around the Arian kind of controversy. The main denominations of Orthodoxy, Oriental Orthodoxy, Church of the East broke off 100-200 years later after enforced over disputes a level less great. So the argument would be over "is Jesus divine and primordial and by how much" rather than "what is the shape of the combination of divine and human nature in Jesus".
If at a later date an Emperor does decide to back Christianity, he might be a true believer without an interest in consensus. Instead of ecumenical councils, his own favoured line is enforced. The first OTL ecumenical councils in Rome (313) and Arles (314) organised to resolve the Donatist schism were failures. TTL councils have the possibility of failing, so that Christian emperor might be pushed into something similar even if his original political aims are different.
Jerusalem would have a smaller Christian population for the time being, because it was patronised as a centre of pilgrimage by Constantine.
I think accruing butterflies of theological factions and the OTL number of later decisions would make Christianity unrecognisable by say the 11th century. That might be true even with a negligible POD, maybe.