[verb (dictionary form)] + に足る / に足りる [noun] + に足る / に足りる
に足る means "worth", "enough to": something meets the bar for trusting, respecting, noticing or proving. It goes after the dictionary form of a verb, or after a する-noun such as しんらい信頼 or ちゅうもく注目, and usually before a noun. にたりる足りる is the same expression in its modern form; に足る is the one you see most.
Both are formal. に足る says something reaches a threshold: it is enough to justify trusting or proving. にあたいする値する says something deserves a response because of its value.
The negative, にたらない足らない or にたりない足りない, means "not worth", and is more common than the affirmative. とる取るにたらない足らない, "trivial", is a set phrase.
に足る belongs to essays, reviews, formal speeches and evaluations. In conversation, a potential verb does the job: しんらい信頼できる, そんけい尊敬できる.