Why were there so many Christian denominations formed with different guidelines or rules instead of just being one church under God and just being Christians or children of God who read the Bible?
Paul answered your question here:
1Co 3:1
And I, brethren, could not speak unto you as unto spiritual, but as unto carnal,
even as unto babes in Christ.
1Co 3:2
I have fed you with milk, and not with meat: for hitherto ye were not able
to bear it, neither yet now are ye able.
1Co 3:3
For ye are yet carnal: for whereas
there is among you envying, and strife, and divisions, are ye not carnal, and walk as men?
1Co 3:4
For while one saith, I am of Paul; and another, I
am of Apollos; are ye not carnal?
1Co 3:5
Who then is Paul, and who
is Apollos, but ministers by whom ye believed, even as the Lord gave to every man?
1Co 3:6
I have planted, Apollos watered; but God gave the increase.
1Co 3:7
So then neither is he that planteth any thing, neither he that watereth; but God that giveth the increase.
Denominations exist because people are carnal or because they are looking to men, and not to God.