If you believe in god, meaning that you have a personal relationship with god, what need do you have for religion?
Isn't it essentially other people telling you how you should have a relationship with god?
I would say, having a relationship with anyone, is bidirectional. So I can't just decide to have a relationship with God, on my own terms - especially as he is sovereign, he has actually more say in the matter, than I do. So it would be my responsibility to find out, what is alright with him.
Otherwise, I may well find myself saying, "I have a relationship with God", when God is saying, "Who are you? I don't know you. Get away from me - be gone."
Like saying, I have a relationship with Jim, and Jim is telling all his friends, "I don't know that weirdo. He be like tripping or something."
Why is Jim not my friend? He doesn't know me. He doesn't like my ways. He doesn't want to be associated with me.
For me, the situation is similar with a relationship with God. See
Worship. I have come to appreciate that the Bible is not the word of men, telling us how we should have a relationship with God... but God actually telling us, through his servants previous to me, how we can have a relationship with him.
Of the many pieces of evidence for this, is the order that exists in creation. I would expect that God would want his people organized as well, and this I have seen - from the ancient Israelites, through to the disciples of Christ, right into our day.