How do I know God exists?
In short: There is no proof that forces faith into your head, and it is a good thing there is not, because then it would no longer be love, it would be compulsion. But there are a multitude of signs pointing that way, the world existing instead of not existing, beauty, conscience, the thirst for meaning. Faith is not a leap into the void, it is a step toward a light you can already glimpse.
The Orthodox nuance
First, think of the simplest thing. Why is there something, rather than nothing? The universe did not have to exist, and yet it does, it has rules, it has order, it is intelligible. Saint Paul said that the invisible attributes of God are seen, understood by the things that are made (Romans 1, 20). Meaning creation is not a mathematical proof, but it is like a work of art that speaks to you about someone.
Then there is something even closer to you, your conscience. Where does this sense come from that some things are truly evil and others truly good, beyond what is convenient for me? And where does this thirst to be loved come from, to have a purpose, which nothing in this world completely quenches? The Fathers used to say that the soul is made for God, and that is why it does not find its absolute rest in anything else.
But be careful, in Orthodoxy you do not know God primarily with your mind, like a theorem. You know Him mostly through experience, through prayer, through the purification of the heart. Blessed are the pure in heart, for they shall see God (Matthew 5, 8). The saints did not believe in God as an idea, they met Him. That is why the best argument is not a clever word, it is a holy person.
Sources
- Romans 1, 20 (creation points to the Creator)
- Matthew 5, 8 (the pure in heart will see Him)
- Acts 17, 27-28 (God, not far from each one of us)