I found a quote which I found to be very inspiring, then looked it up and turns out it was originally said by a Christian theologian. Is there a problem to continue using that quote/ sharing it with others?

We accept truth from all people, yet I strongly suggest discussing each particular quote with a rabbi before you pass it on, to make sure that it's not inadvertently involving "anti-Judaic" concepts without your noticing; and also, don't say it in the name of other religions (which sounds like glorification), but rather just as a general truth (which is probably also found in Jewish sources, as well, for "all is found in the Torah", and why not find the Jewish parallel)!