The Age of Reason by Thomas Paine

The only idea man can affix to the name of God, is that of a first cause, the cause of all things. And, incomprehensibly difficult as it is for a man to conceive what a first cause is, he arrives at the belief of it, from the tenfold greater difficulty of disbelieving it. It is difficult beyond description to conceive that space can have no end; but it is more difficult to conceive an end. It is difficult beyond the power of man to conceive an eternal duration of what we call "time"; but it is more impossible to conceive a time when there shall be no time.

In like manner of reasoning, everything we behold carries in itself the internal evidence that it did not make itself. Every man is an evidence to himself, that he did not make himself; neither could his father make himself, nor his grandfather, nor any of his race; neither could any tree, plant, or animal make itself; and it is the conviction arising from this evidence, that carries us on, as it were, by necessity, to the belief of a first cause eternally existing, of a nature totally different to any material existence we know of, and by the power of which all things exist; and this first cause, man calls God.