Those Who Set The Ranks - 37:4
That your God is one.
In the context of verses 3, 4; all the books of God contain the common message of "One God", hence the reference is to Jews, Christians and Muslims who read the message.
The Cow/2:113
"The Jews said: "The Christians have no foundation,"while the Christians say: "The Jews have no foundation."Yet they all read the Book. The ignorant speak just like them. On the Day of Resurrection Allah will judge between them on the matters on which they differed."
The Spider/29:46
"Argue with the people of the Book in the best way, except the unjust, and say: "We believe in what has been sent down to us and what has been sent down to you. Our God and your God is one. To Him alone have we submitted."