Who wrote the first English statement declaring that the king or any governmental leader had no authority over the religious or non-religious convictions of the people under their rule?

Perhaps shockingly enough given current perspectives, it was a Baptist theologian in England: Thomas Helwys. He bravely wrote a book about religious freedom and dedicated it to King James, who was at that time punishing both Catholics and other dissenters from Anglicanism, like Baptists. For his troubles, he died in jail likely because the stance he took. In 1611, he wrote:

"For we do freely profess that our lord the
king has no more power over their consciences than over ours, and that is
none at all. For our lord the king is but an earthly king, and he has no
authority as a king but in earthly causes. And if the kingís people be
obedient and true subjects, obeying all human laws made by the king, our
lord the king can require no more. For menís religion to God is between
God and themselves. The king shall not answer for it. Neither may the king
be judge between God and man. Let them be heretics, Turks, Jews, or
whatsoever, it appertains not to the earthly power to punish them in the
least measure.
This is made evident to our lord the king by the
scriptures." [emphasis mine]

Al Mohler, President of the Southern Baptist Theological Seminary, describes the benefits to Baptists (and by extension, other Christians) of having a somewhat declining cultural influence and being increasingly out of step with current political and popular trends.

"For all the challenges we will face in the future, this is a great time to be a Baptist. We now have the opportunity to recover our nonconformist roots. That is where we began. We were outsiders, not insiders. In fact, Baptists have always been better when we are outsiders. When Baptist are forced to be nonconformists, we are forced to go back home."

The church in Europe has been on the downward trend for decades. Much of that can be laid at the feet of the intertwined nature of European nations and state sponsored/supported churches. American has retained a religious identity partly because of the seperation of church and state that was pushed for by Baptist like Roger Williams and other Christians working to found the nation in the new world. The areas of the world where Christianity is growing most rapidly are those that have the least cultural trappings of the faith – areas where Christianity is outlawed or certain faiths are dictated by the government. Those nations and regions are taking the reigns of global Christendom.

While Christians in America should always have the right to participate in the democratic process and have the responsibility to do so in light of and at the spurring of their faith, they must also remember that political change will never equal Kingdom change and rarely will it equal individual change.

Christianity, from its founding, works better as an outside influence, changing individuals who in turn impact society for the better, even while the government tries to hinder that influence. There's a reason why Jesus invested His life in common fishermen and not governmental bureaucrats. One is unhindered by the need to please other people. The other, by the very nature of his position, seeks to placate the masses, which means a shying away from anything controversial.

This does not mean the American Christian should avoid political engagement, as the early Anabaptists did in Europe. They should simply recognize the priority level of political activism, while always remembering that our faith calls for a free people with the ability to respond to the Gospel, but never a coerced people repeating a mandated faith statement.