*How can you say that the only truth about God is that He has form?* 

It is undoubtedly true that God comes down to earth in a human form, as in the case of Krishna. And it is true as well that God reveals Himself to His devotees in various forms. But it is also true that God is formless; He is the Indivisible Existence-Knowledge-Bliss Absolute. He has been described in the Vedas both as formless and as endowed with form. He is also described there both as attributeless and as endowed with attributes.

Do you know what I mean? Satchidananda is like an infinite ocean. Intense cold freezes the water into ice, which floats on the ocean in blocks of various forms. Likewise, through the cooling influence of bhakti, one sees forms of God in the Ocean of the Absolute. These forms are meant for the bhaktas, the lovers of God. But when the Sun of Knowledge rises, the ice melts; it becomes the same water it was before. Water above and water below, everywhere nothing but water. 

Therefore a prayer in the Bhagavata says: 'O Lord, You have a form, and You also formless. You walked before us, O Lord, in the shape of a man; again, You have been described in the Vedas as beyond words and thought.'

But you may say that for certain devotees God assumes eternal forms. There are places in the ocean where the ice doesn't melt at all. It assumes the form of quartz.

From: The Gospel of Shri Ramakrishna.