Sometimes the claim of "oldest" can be a matter of interpretation, so let's examine the fine print.
First of all, we know the oldest churches in America are the Roman Catholic Missions founded in the 1500s by the Spanish, many of which are still operating in the West and Southwest. And Saint Luke's Church, aka "The Old Brick Church" in Isle of Wight, Virginia was also founded in 1632, at least according to tradition. I am trying to learn more about its founding, but I'm not going to split hairs. Saint Luke's is an Episcopal Church, not a Congregational Church. Saint Luke's seems to pre-date the church in Jamestown by about five or six years. There were some earlier settlements that had churches, but when the settlements failed, the local churches shut down with them. Saint Luke's Church might even predate our Marshfield church as the oldest Protestant church, depending on the month and date in 1632 that each was founded, though not too many records survive that can pin it down with that kind of accuracy that I know of. However, they're the oldest Episcopal Church I can find, ours is the oldest Congregational I can find. And one of us is the oldest Protestant church in the US, and I suppose we'll never know which one of us that is! We should get together for some Bar-B-Que!
So the first point of the fine print is that we're talking about Congregational churches. And that brings me to the second point of the claim, that is that the church retains its original Traditional/Orthodox Christian beliefs, and remains in fellowship with other Congregational Churches.
Indeed, there are older churches in this area that were originally founded by the Pilgrims or the Puritains, including the church in Plymouth itself (1620), and the First Church in Salem, Massachusetts (1629). But along the way, they rejected Traditional/Orthodox Christian theology and went along with the Unitarian and Universalist movements in the late 1700s and early 1800s. This raises the question as to whether they may still be considered the same church. Can a church that has had such a fundamental shift in its core beliefs and denominational affiliation really be considered a continuation of the original church? Can a an Italian restaurant called "Joe's Eatery," that's bought by a new owner and becomes a Breakfast place but keeps on the waitstaff and keeps the same name still claim to be the same restaurant?
This movement divided many churches. The votes to switch were sometimes very close, and the members who lost and did not approve of the change to Unitarianism or Universalism left those congregations to found new churches to continue in their traditional faith. Sometimes it worked the other way, and the supporters of Unitarianism were the ones that split off of a church voting to retain its traditional theology! Our church in Marshfield, when choosing a new minister in 1837, rejected several Unitarian candidates to choose an orthodox, evangelical minister, and thus remained with the traditional faith.
The Unitarian and the Universalists would later merge and are now known as the Unitarian Univeralists Association. The Congregational Churches would later merge with the Evangelical and Reformed Churches in the US to become the United Church of Christ.
This all left the church in Marshfield as the oldest continuously traditional Trinitarian Congregational church still standing.
As for being the oldest church, I would respectfully suggest that some of the churches making the claim should remember the Catholic missions founded by the Spanish in the 1500s, and also that our First Congregational in Marshfield and Saint Luke's in Isle of Wight, Virginia both have maintained their core beliefs without interruption since 1632 as Protestant Christian churches.
I would also ask about the parties to the split that would have happened in the church when it went Unitarian. Was it the Unitarian faction that split from the church but kept the building and assets? That interpretation is a more materialistic definition of a church. And what of the Traditional/Orthodox faction that moved on to found a new church up the street as a continuation of the original church? Were they keeping the vision of the original founders alive?
Some churches claim to be continuously Congregational because they retain a congregational form of church governance (being run by democratic vote within the congregation rather than by a higher authority such as a Bishop). The Salem church mentions this on its web site. But again, the congregational form of church governanace, is shared by other denominations such as the Baptists, and of course, the Unitarian/Universalists. It's only part of the equation, and it's the belief in a Triune God and the Deity of Jesus Christ that is at question, and when a church drops that belief, it ceases to be a Christian Church, and starts being something else, in most cases, Unitarian Universalist, who regard Jesus with respect and honor, but don't consider him part of the Godhead. It's really a different religion, and my argument is that changing religions ends one church and starts another.
Congregational churches retained their identies and core beliefs when they merged with the United Church of Christ, and there was no break with traditional beliefs or form of government. Only the wider community of Christian fellowship was widened.
It's not my intention to start a controversey, and any church group that would like to press a claim of being the "Oldest ______ in the US," is free to do so as long as they give us the "fine print," as I have here. And based on what I've just explained, we can proudly claim to be the oldest continuously operating Trinitarian Congregational church in the US, and depending on exactly when in 1632 the church was founded, maybe even the oldest Protestant church, although we're not as sure about that particular claim as I've explained above.