Full Idea
We can't infer the existence of God from knowledge of this world, because we should have to know all possible worlds in order to compare them - in short, we should have to be omniscient - in order to say that it is possible only through a God.
Clarification
'Omniscient' means knowing everything
Gist of Idea
To know if this world must have been created by God, we would need to know all other possible worlds
Source
Immanuel Kant (Critique of Practical Reason [1788], I.II.II.VI)
Book Reference
Kant,Immanuel: 'Critique of Practical Reason (Third edition)', ed/tr. Beck,Lewis White [Library of Liberal Arts 1993], p.146
A Reaction
A nice remark, but not wholly convincing. This argument would block all attempts to work out necessities a priori, such as those of maths and logic. Must we know all possible worlds intimately to know that 2+2 is always 4?