Help with Verse Identification -- 'Jesus has not yet returned because...' I have a particular verse in mind that I can't seem to remember. The gist is: 'Jesus has not yet returned for the sake of those who have yet to be added to his kingdom...'

It's something along those lines; if anyone knows what I am referencing I'd greatly appreciate a verse reference.

Thank you!
I believe these are the two main candidates for what you are after:

And this gospel of the kingdom will be preached in the whole world as a testimony to all nations, and then the end will come. - Matthew 24:14 NIV
The Lord is not slow in keeping his promise, as some understand slowness. Instead he is patient with you, not wanting anyone to perish, but everyone to come to repentance. - 2 Peter 3:9 NIV