Well, to start off, if Western churches never united into the Catholic Church, then Islam may be butterflied away. Keep in mind that the Western Church unification for a time meant Eastern Church unification to at least some extent through the end of the 1st millennium, and doing away with that might never allow Islam, or at least not any recognizable similar religion, to ever exist, historically speaking. Islam was certainly not inevitable by the 300s.
You'd have to make Constantine never decide to convene the Council of Nicaea (the easiest and latest decision-based POD, in my opinion), which may be done if he never heard the recommendation to start the council of Nicaea, rejected the idea, or simply ignored it in his daily business. However, this may not prevent an eventual similar council. Such a council, however, would inevitably not be as large, as the emperor did not sanction it, so it may not be enough to prevent eventual unification, if not a greatly weakened one that is less widely accepted.
This would result in the lack of a decline of Gnosticism and Arianism for probably at least a few decades; earlier divergences might allow it to flourish indefinitely.