It’s one of my favourite hymns!
Lucky you to have it at Mass. I have only heard it in a Catholic church about 2 times in 35 years!
Love divine, all loves excelling,
Joy of heaven to earth come down;
Fix in us thy humble dwelling;
All thy faithful mercies crown!
It’s a bit poetical and vague, and is also archaic English, but my understanding is that the singer is acknowledging that Jesus has bestowed many “faithful mercies” (and graces) on him, and is inviting Jesus to dwell in his soul, as the greatest mercy (ie. “crown”) of all.
In normal English, it would read something like:
Live in me, and thus crown all your other mercies.