How should one respond to someone who says, "I believe in the love of the people but not in God"?
I would ask such a person: "What is love?"
Without God, all that exists is the material. In that case, "love" is simply a biochemical reaction in the brain and nothing more. As the character of the devil says in the movie The Devil's Advocate: "[Love is] overrated. Biochemically no different than eating large quantities of chocolate."
In this context love is nothing more than an evolutionary device to make us feel connected to one another and thus enhance our chances of survival as a group. As such it can be logically discarded as unnecessary at this point in human development or perhaps be used as a behavioral tool for calculated survival rates but certainly not as a moral principle. Expenditure of too much time and resources due to a biochemical reaction in our minds would be foolish.
Even if we accept love as a broad philosophical principal, just what exactly is a loving action? Human history is replete with cultural ideas of what is loving and yet can also differ widely.
So what is love, and what does it mean to be loving? Scripture, thankfully, give us the answer:
. . . love one another as I love you (John 15:12).
God is love (1 John 4:8).
In this is love: not that we have loved God, but that he loved us and sent his Son as expiation for our sins (1 John 4:10).