what is the meaning of "if" in this paragraph?

Question

what is the meaning of "if" in this paragraph? what is the description of "if flattish bosom"?

Baxter has written a pamphlet with the portentous title, "Narrative of Facts, Characterising the Supernatural Manifestations, in Members of Mr. Irving's Congregation, and other Individuals, in England and Scotland, and formerly in the Writer Himself." Spiritual truth could no more come through such a mind than white light could come through a prism, and yet in this account he has to admit the occurrence of many things which seem clearly preternatural, mixed up with much that is questionable, and some things which are demonstrably false. The object of the pamphlet is mainly to forswear his evil and invisible guides, so that he may return to the safe if flattish bosom of the Scottish Church. It is noticeable, however, that a second member of Irving's congregation wrote an answering pamphlet with an even longer title, which showed that Baxter was right so long as he was prompted by the spirit, and wrong in his Satanic inferences. This pamphlet is interesting as containing letters from various people who possessed the gift of tongues, showing that they were earnest-minded folk who were incapable of any conscious deception.

Answer 1

Using "if" as a conjunction between two adjectives means "even though":

The movie was an entertaining if predictable ending to the series.

In your case, it is using the metaphor "the bosom of the church" (the church seen as a place of security) but it is playing with this metaphor by describing the bosom of the Scottish church as "flattish" (as a thin woman's bosom may be flattish) Perhaps this suggests that the church is safe, but it is not very nurturing. This is an extended metaphor, and is worthwhile thinking about the layers of meaning.