“What does the subject mean?”

What does “the subject” stand for? Could you help me with this?

The workaday outcome of natural theology was that a country priest can spend much time studying nature. Hence, Darwin’s fairly favorable, if not spiritual reaction to the prospect of entering the church. He recalled, “I asked for some time to think, as from what little I had heard and thought on the subject I had reservations about declaring my belief in all the teaching of the Church of England; though, otherwise, l liked the thought of being a country priest.”