The Universe is all about cause and effect. Every thing has a cause and an effect.

Except God, He has no cause.

This is part of what is meant by God saying "I Am". He is eternal (which is hard to get our limited brains around), He has no beginning or end, in contrast to the universe.

The existence of God is logical:

But how can we know if God exists? And if he does, can we have anything to do with him?

The following quote is from page 322 of the book "Desiring God" by John Piper,

"Suppose I try to go back a million billion trillion years to imagine the nature of original reality. What was it like? What I see is the stunning fact that, on the far side of reality, so to speak, there was a fifty-fifty possibility that original reality was a Person rather than a gas. Just think of it. Since whatever originally was has always existed, there are absolutely no causes that could have disposed that original reality to be a gas rather than a person. Every reasonable person must admit that, from the far side of past eternity, it was, you might say, a toss- up. Maybe some undefined stuff would exist - or maybe a Person! Admitting the reasonable possibility that ultimate reality could be personal has a way of freeing you to consider subsequent evidence more openly. My own inescapable inference from the order of the universe and the existence of human personhood and the universal sense of conscience (moral self-judgement) and the universal judicial sentiment (judgement of others who dishonour us) - my own inference from all this is that Ultimate Reality is not impersonal, but is indeed a Person. I simply find it impossible to believe that the human drama of the centuries, with its quest for meaning and beauty and truth, has no deeper root than molecular mutations."

According to the Bible, God exists and we were created to worship Him.

If we don't worship him, then we worship something else in his place, something created, such as possessions, sex, drugs, money, pleasure, another person and so on. This is what it means to be idolaters and pagans. The only way out of that trap is to acknowledge that there is a Creator.

Creation exists to worship, glorify, honour and praise the Creator and this should be the source of fulfilment and joy for us as created people.

Romans 1:20 "For since the creation of the world God's invisible qualities - his eternal power and divine nature - have been clearly seen, being understood from what has been made, so that men are without excuse."

How do we know if God exists?