The song was written by Tony Asher and composed by Brian Wilson. It was released in 1966 on the group's masterpiece "Pet Sounds" album.
The words are elegantly simple, pure, and straight to the heart as the narrator professes eternal love to a special person:
"I may not always love you
But long as there are stars above you
You never need to doubt it
I'll make you so sure about it"
And then we hear the words:
"God only knows what I'd be without you"
In the second verse, and repeated in the third, the singer tells the lover that life would be empty without him/her in their life:
"If you should ever leave me
Though life would still go on believe me
The world could show nothing to me
So what good would living do me"
"God only know what I'd be without you"
While the song wasn't a great commercial success in the United States, it was very popular in Europe. And the tune has been listed on several "greatest song" lists including Rolling Stone magazine, Rock and Roll Hall of Fame, Pitchfork (No. 1 of songs from the '60s).
There's interesting history behind the song in Wikipedia.
"God Only Knows" lyrics