When we look at how the Unitarian Church gained strength, we encounter a most interesting connection: the influence of the Ottoman Empire. In Transylvania, which was part of the Ottoman territories in the sixteenth and seventeenth centuries, monotheistic beliefs grew very powerful. In a sermon entitled "Islam, the US, and Yeats' Dilemma," Jack Donovan, a priest in the Florida Unitarian Church, emphasizes this development:
In Poland, Hungary, and Transylvania, some Reform Protestant Christians began asserting as a matter of faith, "God is one. There is no god but God." Dangerous heresy in Christendom in those days. Where did dead-defying statement come from?... In 1520s and '30s, when Protestantism was still very new and trinitarian, the Islamic Ottoman Empire conquered Croatia, Hungary, and Transylvania.93

As expressed by many historians and Unitarian clergy, the reason why this monotheistic sect located in Ottoman territories gained in strength was because Islam brought a climate of tolerance. Susan Ritchie of the North Unitarian Universalist Church emphasizes this fact in a sermon entitled The Promise of Postmodernism for Unitarian Universalist Theology:
Most moderate international historians accept not only that the political protection of the Ottomans allowed for the development of progressive Protestantisms, but also that the infamous permissiveness of Ottoman administrative practice regarding local customs and religions must have had some influence with regards to the issue of toleration. 94

Islam's powerful monotheism was an enormous guarantee for anti-trinitarian Christians, for within the Ottoman Empire they could express their opinions freely, enjoy official tolerance, establish their own churches, and reinforce the Christian monotheistic tradition.

The links between Islam and the Unitarian Church have attracted the interest of researchers for hundreds of years. For example, in his The Hungarian Protestant Reformation in the Sixteenth Century under the Ottoman Impact, Alexander Sándor Unghváry concentrates on the importance attached to Islam by Servetus, an earlier proponent of monotheism.95 In his work, based on the relationship between Socianism and Islam, Mathurin Veyssiére de la Croze claims that the Unitarians of Transylvania accepted the similarity between the oneness of God as taught by Unitarianism and that taught in the Qur'an.96
Unitarian clergyman Jack Donovan also draws attention to these matters in a sermon:
Two Islamic teachings would have become common knowledge and would have been much noted. One, the words of the daily call to prayer sung from the minarets to the general public: "God is One. There is no god but God. There is no god but God." And two, the explicit requirement of the Quran, emphasized by Muhammad, that respect and tolerance be given to all religions because each is a response to God. When those teachings are applied to the gospel of Jesus, you get 16th century Unitarianism. It is my hypothesis that our tradition has a 450 year old debt to Islam for a center we share in common…97

Later in the same sermon, Ritchie stated that Unitarian leaders throughout history have always held a positive view toward Islam:
The 17th and 18th century European Socinians were not so shy about praising theological Islam as a pure monotheism that had corrected many of the theological corruptions that had befallen the Christian church since its early days of honest, non-doctrinaire practice. Andrew Ramsey in 1727 spoke if Socinianism approvingly as the sublime religion which stems from "Ideal Islam" (Bastianensen 21). Henry Stubbe, John Toland, Arthur Bury, William Feke and Stephen Nye were similarly all Socinian authors who strategically employed a sympathetic stance towards theological Islam as means of highlighting the deviations from primitive Christian practice that they found bothersome especially in the form of Anglican orthodoxy.98

Mark D. Morrison-Reed of the Toronto Unitarian Church also describes Islam in a sermon entitled The Islamic Connection:
Houston Smith writes that Islam's "innovation was to remove idols from the religious scene and focus the divine on a single invisible God for everyone."[p. 236- Houston Smith, The World's Religions] Unlike Christianity Islam is unmistakably monotheistic, and unlike Judaism was not confined to one people. We might begin any effort to connect with Islam with this: acknowledge that we share common historical ground in this intuition about and understand of God's singularity. In the Middle Ages it was Islam tolerance that allowed a cultural bridge between Christianity and Islam to develop. This Spanish Renaissance influenced a person we claim as our intellectual forebear, Michael Servetus. Servetus was born in 1511 in northern Spain and while we know some of the details and influence upon his life, we don't know exactly how his ideas developed or what precipitated the publishing in 1531 of his book On the Errors of the Trinity… While Islam had created the political and intellectual conditions that contributed to the emerging of Servetus' ideas in the West, it was also responsible for the political conditions that allowed Unitarianism to germinate, blossom and spread in eastern Europe… In a sense we are indebted to Islam. For me that suggest that we need to stop viewing Islam as something foreign and incomprehensible. Instead, it is time to recognize that not only are we historically connected but that we share some common values, as well.99
These statements of different Uniterian clergymen reveal the climate of tolerance in Ottoman territories and the common values shared by these two revealed religions.