Solas Centre for Public Christianity, which has links to the Free Church of Scotland, said humanist organisations wanted to "remove all traces of Christianity from Scotland's state schools".
The group is headed by Free Church Moderator Revd David Robertson and will take its campaign to the Scottish Parliament later.
Revd Robertson will say: "We are concerned at the increasing pressure on Scotland's education system to move away from its Christian ethos and roots.
"Whilst other progressive nations such as England, the Netherlands and Sweden allow for a variety of education and faith schools, it appears that the Scottish system is seeking to be a one size fits all which increasingly marginalises Christianity.
"We call upon the Scottish Government to resist the continuing pressure from secularist groups who are seeking to remove Christianity from the State education system.
"We support the existing faith schools such as the Roman Catholic and Anglican schools and would encourage the government to develop and assist more faith schools.
"Scotland could lead the way by democratising our education system by allocating vouchers to parents to produce financial support for faith schools.
"A diversity of schools would create a healthy competition of ideas and outcomes and improve educational standards and the faith debate across the nation."