John Adams was raised a Congregationalist, but became a devout Unitarian Christian as an adult.
As one of America's Founding Fathers, he advocated for the system of checks and balances that is integral to the U.S. constitution.
John Adams was born and raised in Braintree, Massachusetts (now called Quincy). He died in 1826 at the age of 90.
Adams came from a line of Massachusetts Puritans and grew up as a Congregationalist, but along with many other Congregationalists of the time, he became a Unitarian as an adult. He and his wife Abigail attended the First Parish Church in their hometown, and were active members throughout their lives.
Along with his fellow Unitarians, Adams rejected the concepts of the Trinity, the divinity of Jesus, and the inerrancy of the Bible. But he respected the right of all Christians to worship as they pleased.
Adams agreed with his fellow Founding Fathers in the separation of church and state, mostly due to his belief that governments corrupt religion. If pastors, for instance, are paid by the state, they don't owe anything to their congregations. But he wasn't as suspicious of religion as some of his contemporaries like Thomas Jefferson. In a letter to Jefferson, he wrote,
Without Religion this World would be Something not fit to be mentioned in polite Company, I mean Hell.
In fact, he issued calls to prayer during his time as president. When war with France seemed imminent, he released a proclamation asking for a day of "Solemn Humiliation, Fasting, and Prayer." He implored his fellow Americans to humbly ask God for "his inestimable Favour and Heavenly Benediction."
Adams was clearly a devout Christian, and thought it an important character trait in his fellow humans. In responding to Thomas Paine's criticism of Christianity, Adams wrote,
The Christian religion is, above all the religions that ever prevailed or existed in ancient or modern times, the religion of wisdom, virtue, equity and humanity.
The Grandest, Noblest, and Most Useful
For a man with an affinity for political theory, John Adams was born to the right time and place in history. In his diary he wrote,
[Politics are the] divine science, the grandest, noblest, and the most useful . . . in the whole circle [of sciences].
He was an ambassador to France and Holland during the Revolutionary War, helped draft the Declaration of Independence, served as the first vice president for two terms under George Washington, and served one term as the second president of the United States as a member of the Federalist party.
Although he wasn't present at the Constitutional Convention, Adams advocated for the system of checks and balances that became a central part of the constitution. While most Americans saw danger mostly in the executive branch, Adams saw it from all corners. He wrote,
The fundamental article of my political creed is, that despotism, or unlimited sovereignty, or absolute power is the same in a majority of a popular assembly, an aristocratical counsel, an oligarchical junto and a single emperor.
During his term as president, Adams was confronted with a hostile France who disapproved of his election over Thomas Jefferson. After the European country began attacking American ships, many citizens, especially those in his own party, were calling for a declaration of war. Adams instead negotiated a treaty with France, which eventually cost him his reelection, and ended his political career.
Adams is remembered more as a political theorist than as a politician, but his place in the collective American psyche is as cemented as his peers, Washington, Jefferson, and Madison. If you want to learn more about this Founding Father, his Wikipedia page is a good place to start.