I am a Christian who read both Lydia Whipple's and Susan Eddy's letters, but I cannot write an affirmation to the sentiments they expressed, as was requested.
The rights guaranteed by the First Amendment would seem to be threatened more today by those who claim to follow Christ rather than by secularists. Claims are made that are simply untrue.
Christians do have a right to pray in public, to pray in school and to worship (or not) as they see fit. What courts have (appropriately) deemed inappropriate is to employ the power of the state to promote or enforce any sectarian viewpoint or practice.
This is the whole point of the First Amendment (and the Bill of Rights) in general: To protect the minority from the tyranny of the majority, as incorporated in the state. But it is demagoguery to claim that Christians don't have rights to worship or express their beliefs.
Furthermore, to claim that the Constitution doesn't use the phrase "wall of separation of church and state" is to state something true but miss the larger point. Thomas Jefferson, a Founding Father if there ever was one, used this phrase to describe the purpose of the First Amendment. The "Father of the Constitution," James Madison, endorsed this same view dozens of times in his many writings. Why some refuse to accept Jefferson's and Madison's many clear statements on the First Amendment is beyond me.
Our country is unique in world history -- the first government in which the power of state and church were (providentially?) not intertwined. Through all this world's sad history the power to compel the conscience and enforce religious practice has been a regular feature of political power.
Julius Caesar for a time held the office of Pontifex Maximus (most high priest) in Rome. During many centuries ambassadors between governments were usually churchmen.
And in Europe today, where official churches established by their respective governments hold sway, religious practice is comparatively weak. Many outside observers have noted a causal connection between the First Amendment and America's tradition of religious fervor.
In all this I can't help think of Jesus' words to Pilate: My kingdom is not of this world. If my kingdom were of this world my followers would fight (organize a political party?) but now my kingdom is from another place.
I too pray for Christian unity, a unity through which hearts and minds are convinced to follow Jesus because of the love His disciples have for each other and all humanity -- especially those with whom they disagree, and not by Christians winning battles -- military, political or otherwise.