Although John Adams was born to a Congregationalist (Puritan) family with a Deacon for a father, he was an independent thinker who converted to Unitarianism in his adult life. Was John Adams a Christian? Yes, at this time Unitarian Universalism had yet to develop into the open, non-denominational format that it takes today and John Adams' religion was very much rooted in the teachings of Christ. However, John Adams religious beliefs were certainly controversial at the time because Unitarians did not adhere to the concept of the Holy Trinity nor did they believe that Jesus Christ was necessarily divine. Unlike most Unitarians, Adams did believe in the miracles of Jesus Christ. Historians have debated whether or not John Adams' religious views were influenced by the growing movement of Deism, an ideology that believed in an Intelligent Creator but did not believe in miracles or divine intervention of any kind in human affairs. Notable Deists of the time include Thomas Jefferson, a friend (and later, political rival) of Adams who put together his own version of the Bible by editing out all of the miracles and non-historical information. However, Adams' pronounced belief in miracles and the value of regular church attendance make it unlikely that he gave much credence to the Deists.

There are many famous quotes of John Adams on religion, especially in response to the Atheism of Thomas Paine. For John Adams, Atheist beliefs were a threat to a decent and moral society. He rebuked Thomas Paine's criticism of Christianity by declaring that no other religion had more "wisdom, virtue, equity and humanity." But John Adams was independent of mind to recognize consequences of any established religion. In the view of John Adams, Christianity had been twisted over the centuries by authorities who used superstition and division to control the populace, abuse minorities, and lead large scale wars. In the writing of John Adams on religion, he often criticized the Roman Catholic Church for its corrupted structure of power and deceit. John Adams' religion certainly changed during his life, but he always believed in the virtue of Christianity and attended church regularly throughout his life.

Despite his assertion that religion had a role in public life, John Adams was most definitely in favor of separating Church and State. He did not believe that religious views should either hinder or help a politician in matters of law and politics which needed only reason and common sense. He believed that allowing for free conscience would allow men of all religious beliefs to succeed in uniting together for the good of society and the state. Even though he had plenty of contempt for Catholics and even the Jesuit priests who came to America in increasing numbers, he recognized that the nation must accept them on principle of religious freedom. Adams believed that the freedom of religion granted by the state would be the final death blow to all corrupt forms of religious authority. Many of his thoughts on religion can be found in the hundreds of letters he wrote that were saved for posterity.