The row will come to a head at an employment tribunal next month and is being seen as a test of the church's policy that only practising Catholics should hold senior posts in its schools.
Mr McNab said he has repeatedly been the victim of religious discrimination in 15 years and has been overlooked for promotion four times.
He is funding the legal challenge and fears the bill could be more than Pounds 15,000. Mr McNab intends to invoke the Employment Equality (Religion or Belief) Regulations 2003, which ban discrimination on faith grounds.
His supporters say the move will determine whether RC schools can legally bar non-Catholics from holding senior posts.
Mr McNab applied for a job as head of pastoral care at St Paul's RC high, Glasgow, after leaflets were put out seeking staff candidates.
Although Catholic schools state that heads and deputies must be believers, Mr McNab said there was no such stipulation on the pastoral care post. But after applying for the job, which would have resulted in a pound;9,500 pay rise, the married father-of-one said he was told he would not be interviewed because he was not a Catholic. "I am deemed good enough to work in a Catholic school but not get promoted," said Mr McNab. "I am sick of being treated like a second-class citizen. It is like taking a step back into the dark ages. All I want is equality in the workplace and to be treated in the same way as my peers."
Glasgow city council, which will represent the school at the tribunal, said it would not comment on individual cases. But the Catholic Education Service said the employment directive was not binding on RC schools and the case will have no bearing on future policy.
Last week The TES told how RC schools in England were facing a leadership crisis and were far less likely than other maintained schools to appoint a new head after advertising a vacancy. But the church says it will not relax its stance on only practising Catholics holding senior leadership posts.
Terry Sanderson, vice-president of the National Secular Society, said: "It is scandalous that someone's religion can stand in the way of promotion."