Colin Emerson, the vice-chair of Edinburgh Secular Society (ESS) and Norman Bonney, its honorary president, are due to give evidence at the Scottish Parliament.
They want the law which makes it compulsory for three representatives from religious organisations to sit as members of local authority education committees to be scrapped.
The Holyrood session follows a petition submitted by the society to MSPs last year calling for a repeal of section 124 of the Local Government Act 1973.
Under the Act all three of the appointed religious representatives have votes equal to those of elected councillors.
They help shape policies affecting state primary and secondary schools, and nurseries.
The campaigners say that due to the provisions in the legislation, churches currently hold the balance of power in nearly two-thirds of Scottish council education committees.
They say the act, which says one representative on the committees should come from the Church of Scotland, one should come from the Catholic Church and a third from any religious organisation, reflects the historical role churches had in the education system in Scotland.
But they believe it is now out of step with society.
In a submission to MSPs on the Petitions' Committee, the ESS claims that as a result of the law both the Church of Scotland and the Catholic Church have too great an influence in proportion to the number of Scots who share their beliefs.
They say that in the last census in 2011 37% of the population - the single biggest group - said they had no religion.
Around 32% said they belonged to the Church of Scotland and 16% said they were Catholics.
The ESS also pointed out that of the religious members on the country's 32 local authority education committees just two are Muslim and one is Jewish.
This, they say, means the law puts non-Christian religious organisations at a disadvantage.
A Scottish Government spokesman said there were no plans to change the existing legislation.