Descartes had strong ideas about God’s existence. He believed this mainly because of his method of questioning everything and his idea that some things are clear and certain.

One important part of Descartes' thinking is the phrase, "Cogito, ergo sum," which means "I think, therefore I am." This means that the very fact we think shows we exist. Starting from this idea, he took it further to explain why he believed God exists.

In his work called "Meditations," Descartes shares several arguments for God’s existence. One major argument is that we all have an idea of God as a perfect being. He believed this idea couldn’t come from ourselves, who are imperfect. Instead, it must have been given to us by God. So, if we can think of a perfect being, then that perfect being must exist to create that thought.

Descartes also argued against the idea of a deceiving God. He claimed that God wouldn’t trick us. Because of this, he felt that the things he clearly understood couldn’t be doubted. A perfect God wouldn’t let us be wrong about the most important truths. This belief helps support Descartes' confidence in knowledge. He thought that since God exists, we can trust our reasoning and feel sure about what is true.