I rented a house 4 months ago and gave the deposit they required of 3 months/rent. They never mentioned that I should buy rental insurance or there is nothing related on my contract that I should. And it says clearly that "Lessor has obtained insurance to cover fire damages to the building itself and liability insurance to cover certain personal injuries occuring as result of defects or lessor's negligence" NOW they are asking and saying it always has been their policy. Am I supposed to provide insurance?

For starters the owner is the lessor. You are the leasee.

It doesn't matter in this case, but if you sign a contact, yes, you are bound by the terms of the contract.

If the lease does not require that the tenant obtain renter's insurance then you are not required to do so. However if there is fire or other damage the landlord's insurance will not cover any of your possessions. Tenants should always obtain renter's insurance to cover their own property regardless of whether it is required by the lease terms.

Renter's insurance is cheap and you would be foolish to not have coverage. But its not required unless written into the lease.