This is a very beautifully written piece of music, very meaningful and carefully pieced up. Soo Young being christian, perhaps she is referring this love to God's love, guess the lyrics works both ways (to a bf/husband's love as well) anw.
This was my first thought as well when I read the title, because of a famous, christian song called "how great is your love."
I think this verse suggests it the most:
hope that a love like the sky will happen for me too, I always waited for it
On a day when the stars are shining, we walk in rhythm to the birds' melody
Like cyclic seasons' repeating secrets
Your love that never changes makes my eyes shine
Because a phrase like "Your love that never changes" sounds like something straight out of a christian song, but I don't know, it'd be interesting if Sooyoung has said anything about the song herself.