There are some jobs where the outcome is the point, and others where having a person actually do a thing is the point. Priest is very much the latter sort of job.

Forget about AI for a moment and let's consider a more mundane tool, like an industrial robot. Is it OK to use a robot to perform some step in assembling a car? Certainly. How about programming that robot to perform Communion? Not so much. Not because using a robot to do things is inherently immoral, but because the human priest is supposed to be an integral component of Communion, it's not just a matter of transporting a cracker and some liquid into people's mouths by whatever means you choose.