Ron Williams is the father of four children at Darling Heights State School in Toowoomba, Queensland. Like many other schools, it has a chaplain funded by the federal government. Williams took objection to this, saying taxpayers should not pay for a religious position in a state school.

Williams has taken the remarkable step of fighting for his beliefs all the way to the High Court. On Monday, his legal team will argue before the court that the chaplaincy scheme is unconstitutional.

The case is the most anticipated of the year. If Williams succeeds, the court may not only strike down the chaplaincy scheme, but also hundreds of other federal programs worth billions of dollars. The result could rewrite the book on how the Commonwealth and the states spend public money.

The battle against the chaplaincy program has spanned several years. Williams first challenged the scheme in the High Court in 2010. He argued then that the program breached the separation of church and state and, in particular, section 116 of the constitution which says "no religious test shall be required as a qualification for any office or public trust under the Commonwealth". This argument failed.