So he says, “It is desirable for the Truth to be exalted in His apparent name, which is His manifestation in the form for His servants, so He frees Him from being restricted by it, but the exaltation that does not remove Him from knowledge is that He has identified that form, for He is more knowledgeable of Himself than the mind about it and of every knower other than Him through it, and He has said about Himself that He is the One who manifests Himself to His servants.” In that form, as mentioned by Muslim in his Sahih,

So his purity in that case is that he is not bound by a form, that is, he is not restricted by a form, but rather he manifests himself in whatever form he appears.