It's not that he thinks he can delay forever. Sooner or later, Joan is going to catch the attention of a priest of Iomedae, or one of their agents, and he'll either be close enough to orthodoxy to get by, or he— won't.

But maybe he can delay long enough that Joan gets distracted with the sort of heresies that aren't quite as dangerous, or gives up the interest in theology to focus entirely on wizardry, or finally manages to understand that sometimes you believe something because Church and Crown says to, even if it doesn't entirely make sense.

...The good news is, Westcrown seems to have an awful lot of pamphlets advocating all kinds of bizarre ideas that he doesn't think the Church actually condemns. He sorts through a stack of them, trying his best to exclude anything that isn't safe for Joan to believe, and packages the rest with a letter to Conesa. Some of them are almost certainly heretical, but there are kinds and kinds of heresy, and he doesn't think Iomedae would command her priests to kill someone for thinking she accomplished more impressive deeds than she's actually managed.