Glasgow, in common with more than half of education authorities, operates a policy of "reserved" posts at denominational schools which have to be filled by candidates approved by the Catholic Church. These included heads, deputes, principal guidance teachers and religious teachers.
In David McNab's case, pastoral care teachers were not covered, but the council claims they replaced guidance teachers, who would be expected to follow the Church's teaching when dealing with sensitive issues such as sexuality, pregnancy, contraception and abortion.
Catholic representatives argue, in any case, that the system of reserved posts is flawed.
Michael McGrath, director of the Scottish Catholic Education Service, told The TES Scotland following the tribunal's verdict earlier this year that it meant, in effect, that all teachers applying for posts in denominational schools had to be approved by the Church.
"This does not mean they have to be Catholic," he said, "but they have to be approved."
Mr McGrath suggested that decisions on approval would depend on "the extent to which the person is giving public witness to their views and beliefs".
Mr McNab made no secret of being an avowed atheist.