God loves each of us as if there were only one of us.Attr. St. Augustine
This comes in various paraphrases, but what they all have in common is that they aren’t quite what St. Augustine said (the level of quiteness varies from paraphrase to paraphrase). Someone has done most of the legwork for me in this blog post.
The saying appears to originate in St. Augustine’s Confessions, 3.11.19, where he says this:
O thou Omnipotent Good, thou carest for every one of us as if thou didst care for him only, and so for all as if they were but one! (Tr. Albert C. Outler)
In Latin, that’s o tu bone omnipotens, qui sic curas unumquemque nostrum tamquam solum cures, et sic omnes tamquam singulos
I leave it to the reader to decide whether or not the saying at the top of this post is an acceptable paraphrase.