Montaigne's father handed him a large book written by Raimond Sebond and asked him to translate it from the Latin it was written in. A short time later, his father died. Montaigne took the wish to translate it to heart and did so.
The book was on theology and it had gone in and out of favor with the church over time. It sought to prove various theological beliefs through rationalistic means. It had been attacked by other rationalists. Montaigne took it upon himself to defend the writing. (An 'apology' can be thought of as a 'defense'.) He defends Sebond from rationalistic attacks by suggesting that rationalism is always limited, especially in matters of faith. In short, his 'defense' also negates Sebond completely.
I'm not displeased with this approach, though I'm not entirely convinced either. I do believe that there are areas of theology and faith that can't really be grappled with by straight reason. However, I don't know that this is really a limitation of reason so much as just an area outside of its expertise. Mathematics won't tell me which cheeseburger tastes better, but that doesn't mean that it's useless.
This is a long piece and I don't know that I'd recommend it that highly. Montaigne is a wonderful and talented writer but, well, that's more clear in shorter pieces than this. In fact, I've got a Treasure of Montaigne that skips it all together. If you're interested, I thought that the pieces from Year One were stronger and more interesting.