You are either angry at all Atheists, all Christians, or else you are sadly misinformed, for there is no indication that the Founding Fathers had any notion of founding a 'Christian country.'
Having escaped from the legal clutches of the English Established Church and its tentacles in polity and legislation or from similar controlling ecclesiastical powers in Europe, they sought to establish a country where every man would be free to follow the religion of his choosing, or none at all, and to this end provided in the Constitution a resolution that the US Government would keep its nose out of matters religious.
That it has not done so is a matter of record, and that certain cults on the far right of the Evangelical Political Movement has no intention of ceasing from their delusions that it is going to be a 'Christian Ciountry' whether the rest of the county likes it or not, and do so especially in regard to those who are either not Christians, or who are Christians but hold the Constitutional opinion that government should not interfere with the religious and spiritual lives of the people, nor with whatever way they seek to express their beliefs or non-beliefs.
Religion has often been used as a method of social control, and those that advocate the politicising of Christianity do so at the cost of alienating themselves from the teachings of Jesus Christ who said,
"My kingdom is not of this world; if it were my servants would fight."
And those that seek to Christianise politics alienate themselves from God and Jesus Christ by way of distorting their gospel and forcing it, like Procrusteas, to fit whatever object of political philosophy that they decide it must be applied.
Jesus gave other advice that such pseudoreligio-political animals conveniently overlook,
"Render unto Caesar the things that are Caesar's, and unto God the things that are God's"
There is no confusion, no mixing of principles, no borrowing 'across the aisle,' so to speak, but a clear definition of the God given separation of polity and religion. Accordingly, there should be no confusion here, at least for true believing Christians or thinking atheists, etc.
The withholding of the civic franchise from women is not an atheistic principle, but was the natural result of Judeo-Christian patriarchalism that saturated the 'civilised' world.
Likewise, slavery is not atheistic, but it also has Judeo-Christian roots, and while there were some brave Christian ministers and politicians that stood against slavery, there were many more Christian gentlemen that not only exploited the men, women, and children stolen from their homelands and brutalised by their capture, transport, and treatment when bought and enslaved further by decent Christian gentlemen plantation owners that could, at best, only be called Christians up to a point.
Governments ought not to attempt to control what people think about, imagine, or espouse, for such activities by either governments, politicians, religious leaders, and criminals fall inside the much vaunted 'freedom of the individual' to pursue his or her own destiny. It has been referred to as 'The American Dream.'
Although the Constitution does not contain the Decalogue, it is based on the principles of respect and responsibility for the community and family that are addressed in the Decalogue.
The framers of the Constitution were intelligent enough to know that there are some things that cannot be legislated, of whose number are unkindness, hostility, and bad manners. No doubt you will know of others.