As human beings we consist of both body and soul; both of them are marks of God's goodness to us. The body is visible, and the soul is invisible. It follows that we should render to God both visible and invisible service. Outward religion is visible service, and involves such things as praying, fasting, acts of charity, building temples, celebrating festivals, and so on. Inward religion is invisible service, and involves such things as: acknowledging the unity of God in our hearts, loving God in our hearts; being humble towards him, and wishing to obey his every command; entrusting ourselves to him; abstaining from what he hates, and embracing what he loves; and meditating on his blessings.
Bahya: Guide to the Duties of the Heart