Can you explain what you're referring to about the heresy here? My impression is that Christianity traditionally taught that natural theology is possible but is incomplete, in the sense that people could rationally conclude that God exists, but that they would not learn "enough" about him without revelation.

Many Christian theologians attempted to demonstrate the existence of God rationally, so I don't know what about that process would have been considered heretical in its own right. I'd agree that the claim (associated with Deists, for example) that one could have a complete religion based exclusively on reason with no revelation, or that all purported divine revelations are untrustworthy, would have been considered heretical.