Full Idea
Kant speaks of God not as something known or proved to exist by virtue of rational argument, but as a postulate of moral reflection (that is, of 'practical reason').
Gist of Idea
God is not proved by reason, but is a postulate of moral thinking
Source
report of Immanuel Kant (Groundwork of the Metaphysic of Morals [1785]) by Brian Davies - Introduction to the Philosophy of Religion 9 'Morality'
Book Reference
Davies,Brian: 'An Introduction to the Philosophy of Religion' [OUP 1993], p.175
A Reaction
Presumably it is a necessary postulate, which makes this a transcendental argument, surely?