One of the interests in reading a book of this age is to get some idea of the way people thought in those times. Given that so much has a apparently changed since then, it can sometimes be surprising to notice what hasn't changed.
In chapter 19 the narrator and Arthur agree that the country church seemed a better representation of a "house of God" than the more formal city churches. I remember very similar remarks made about the country church I attended as a child, and the expression of city church services a "performances" is also familiar to me - although I don't remember the phrase "blatant little coxcombs" being used.
Also in chapter 19 is a discussion about whether giving (self-sacrifice) is truly good/noble if the gift is only made in the expectation of some later return (as being offered by the church "a thousandfold"). Though this is couched in religious terms in the book, it is a more general philosophical question.
Selfishness and selflessness seems to be a theme overlaying both the contemporary and the Outland settings of the work. There is, by the end of the first book, no sign that selflessness is particularly rewarding, but then that would be consistent with the idea expressed ch19: it wouldn't be selfless if it was rewarding.
In chapter 21 we see the scene with Sylvie and the dead hare. This is a reiteration of Carroll's thoughts from the preface concerning good hunters vs bad hunters. Of course this idea is expressed in religious terms, but again it is not necessary to be religious to have at least some sympathy with the idea that such careless and purposeless killing is wrong. I can't say that I can really see this as theme of the work, though it would appear to have been strong in Carroll's mind at the time.