This may be the hymn included in the work "Stainer's 'Crucifixion' " by John Stainer. The lyrics start off:

God so loved the world, God so loved the world,
That He gave His only begotten Son,
That whoso believeth, believeth in Him,
Should not perish, should not perish but have everlasting life.

etc.

It is slow and majestic - and it is a choral work!

I sang this when I was in the church choir as a child.
I think the lyrics are:
God so loved the world....
God so loved the world...
That he gave...his only son...

There is one more line and then the song is sung in harmony, like half the choir would start singing after the other half had already sung the first line. It is sung very very slowly, and I believe these are the only three lines! I remember that the word "only" is really stretched out, so it's like four syllables. It's a really beautiful song and I can't find it anywhere, there's another song with the line "God so loved the world" but it's a different tune and there are more lines. Does anybody know what this song is and where I can find it?