Shiva Vishnu are One

On his way from north India to the south,Agastya, with his wife, reached Kutralam, in Tamil Nadu. There was a Vishnu temple there. As Agastya was a Shaivite, he was not allowed to enter the temple. Agastya used his miraculous powers and changed the idol of Vishnu in the temple into that of Shiva Lingam and thus demonstrated to the people that Vishnu and Shiva are not different deities, as God is only one.

https://www.google.com/search?q=Kutrala+Nathar