Serving God is incomplete if it doesn't lead to serving others.
"The service of the God of Israel and total concern for life within our horizon are inseparable... His people can truly give all their attention to him without being lured away from their neighbors."(G. C. Berkouwer, A Half Century of Theology, p. 193).
Serving others need not lead us away from serving God.
"It is ridiculous to suppose that the Old Testament is guilty of being too heavily accented and one-sidedly concerned with the horizontal dimension of life, as though love for God might somehow get shortchanged by it." (p. 193).