Think of it as a choose-your-own-adventure novel: Tristram sends his reader back to read the previous chapter again, to find where he said that his mother was not a Papist (Roman Catholic).
When the lady returns, he asks her if she found it. No, she says. It was in the last line: It was necessary I should be born before I was christened (1.19.14).
Still confused? In a footnote, Tristram explains that Roman Catholics allowed a child to be baptized as soon as a part of its body could be seen—and, after 1733, could even be baptized by injection before birth.
Tristram warns his readers that they've really got to pay attention, because a lot of information is conveyed in subtle hints.
He then presents the proceedings of the Sorbonne convention that discussed baptism by injection. In French. Without a translation. In response to all this debate, Tristram proposes a solution: let's baptize the homunculi (the sperm) with a small injection pipe.