This is lovely! I like it without the black zig-zag around the colors. I think that as it is makes it look like the colors of the rainbow radiating out of the cross. I suggest that you make a sample of the colors and see if you like the black on them. I recently did this after much fussing decided not to do any think around my applie. It is easier to do any testing on a sample piece then on the real quilt.
As for borders- none. Just bind it in dark purple, black or maybe gold. No border needed. Batting?_ I use the same batting for my wallhangings as i use for my quilts- Hobbs 80/20.
Backing?- check Hobby lobby (on line if there is not one by you). The might have some fabric with a religous theme.
good luck and show us the results.