This quote is getting at the nature of different kinds of love. At the time of this quote (chapter 11) the main character, Frederic Henry is wounded and in the hospital. His priest-friend comes to visit him. As they talk, the priest notes that Henry does not love God. Henry agrees and says, “I don’t love much.” Then the priest utters the quote you are asking about.
So one kind of love is the love of God. But that’s not all they are talking about. Look at the wording the priest uses, “sacrifice” and “serve.” We often use those words in connection with the love of one’s country, which sometimes involves causes like fighting in a war. This is the second kind of love, patriotic love. But, based on the conversation Henry and the priest are having, neither particularly care for the war or whatever cause is being fought over.
When Henry says “I don’t love much” he is foreshadowing, ironically, what’s about to happen. His relationship with Catherine is about to blossom, and he will grow to love her very much. This is the third kind of love, romantic love. So the quotation and the idea of “serving” and “sacrificing” for something will now apply to Henry’s relationship with and feelings for Catherine, instead of love for God or love for country. In fact, he will throw off everything else to be with her.