I can't decide which meaning of the verb mark is involved here.
As always, it's a passage from god is not Great by Christopher Hitchens:
If religious instruction were not allowed until the child had attained the age of reason, we would be living in a quite different world. Faithful parents are divided over this, since they naturally hope to share the wonders and delights of Christmas and other fiestas with their offspring (and can also make good use of god, as well as of lesser figures like Santa Claus, to help tame the unruly) but mark what happens if the child should stray to another faith, let alone another cult, even in early adolescence. The parents will tend to proclaim that this is taking advantage of the innocent. All monotheisms have, or used to have, a very strong prohibition against apostasy for just this reason.
I can't decide whether mark is attached to the verb hope to or used in the sense as in mark my words. I strongly assume it is attached to hope to but can't be sure as I read the rest of the passage.
Thank you very much.