It may not be possible to prove God's existence like one proves a scientific fact, but it is more rational to believe in God's existence since it is based on evidence. Here's the ontological argument:

God is defined as a maximally great being.

1. It is possible that a Maximally Great Being exists.

2. A Maximally Great Being exists in some possible world.

3. If a Maximally Great Being exists in some possible world, then it exists in every possible world.

4. If a Maximally Great Being exists in every possible world, then it exists in the actual world.

5. A Maximally Great Being exists in the actual world.

6. Therefore, a Maximally Great Being exists.