Undertale is (probably) an accidental theological text and the statements it makes broadly fall into 3 categories: 1 Surface level takes on pacifism and positive sum interaction 2 A bizarre attempt to reclaim "non-christian" occult symbolism which is hermetic/otherwise Christian
3. An unconscious realization that all prophets are false prophets all the way back to Christ himself I could be misreading it catastrophically but this is the basic outline of my interpretation, which I want to write about at book length but have not yet for various reasons.