After the Fall of Constantinople in 1453 to the Turks, Nicholas of Cusa (c.1400-1464) wrote De pace fidei (On the Peace of Faith), in which he envisioned a conference in Heaven of representatives of all religions, including Islam and Hussites (the followers of Jan Hus were one of the first Christian Protestant movements, over a century before Luther).
The imaginary conference concluded that it was possible to have a single unified religion, even if it manifests in many different versions with separate practices. After all, the Roman Catholic Church had co-existed with the Eastern Church for centuries; the Eastern Church may not have recognized the authority of the Pope over the East, but the East and West did consider themselves two parts of the same faith.
Nicholas clearly prefers and exalts Christianity, but is willing to find accord with others. He had written Cribratio Alchorani (Sifting the Koran), which acknowledges that Islam and Judaism still possess seeds of the truth. Let me be clear: Nicholas doesn't treat Islam or Judaism as equals with Christianity—in 1451 he had used his authority as a bishop to require Jews in Arnhem to wear badges, and he had imposed other restrictions on Jews that were later lifted by Pope Nicholas V. The fact, however, that a well-known Catholic prelate and respected theologian could write on and publish such tolerant ideas was remarkable for the time.