Men will not understand that when they fulfill their duties to men, they fulfill thereby God’s commandments; that they are consequently always in the service of God, as long as their actions are moral, and that it is absolutely impossible to serve God otherwise.

Immanuel Kant (1724-1804) German philosopher

(Attributed)
    (Source)

Quoted in Karl Hillebrand, Lectures on German Thought, Lecture 5 "The Triumvirate of Goethe, Kant, and Schiller (1787-1800)" (1879)