"More often, it's the trust that was betrayed, not time." "that ..." in this sentence is not a relative clause but a "It is ... that ..." pattern, which is a different construct. This is sometimes called an "emphatic pattern/sentence" or just an emphatic "that". It is a way of separating out and emphasizing a part of a sentence. Here's an example: ORIGINAL: I love you with all my heart. 나는 너를 진심을 다해 사랑해. 1. It is I that [] love you with all [his] heart. 너를 진심을 다해 사랑하는 사람은 나야. 2. It is YOU that I love [] with all my heart. 내가 진심을 다해 사랑하는 사람은 너야. 3. It is WITH ALL MY HEART that I love you []. 내가 너를 사랑하는 것은 진심을 다해서야. As you can see, different parts of the sentence can be pulled out to the front after "It is" for emphasis. With the original sentence: - More often the trust was betrayed, not time. (in an ordinary sentence) => It's more often the trust that was betrayed, not time = More often, it's the trust that was betrayed, not time. (emphatic) => 더 빈번했던 것은 시간보다 신뢰가 배신당하는 것이었다. => 빈번히 그것은 시간의 문제가 아니라 신뢰의 배신이었다.

I guess this is an emphatic sentence. It is(was)...that... You can make an entire correct sentence after deleting the " It is(was)...that..." The sentence is used to emphasize the word between It's and that.