I'd call it idolatry. When done in public by a Pope, it amounts to blasphemy and apostacy. To be merely indifferentism, he would have had to have given equal attention go the holy books of any number of false religions. The only possibly mitigating circuмstance is the Holy Father was laboring under insanity or gross deception of the devil. When St Paul spoke of "becoming all thing to all men" he meant in order to convert their souls to Christ, not to do so for any intrinsic value. One may eat what he finds a disgusting meal so as not to drive a soul from Truth. But one may never sin against Truth to gain a convert. Then two souls will be lost! Did Gideon worship his father's idol in order to gain him back to the Lord God? Did St. Polycarp burn insence to the apparent idol so as to gain favour with God's enemies? No, death before sin!
It is a scary thought that JP II has already answered to his Judge. Even his canonisation in April will change nothing as pertains to the state of his soul.