(RNS) A secular watchdog organization is suing a congressional chaplain for blocking one of its leaders from delivering an invocation before the U.S. House of Representatives.

The Freedom From Religion Foundation, a Wisconsin-based group that promotes the separation of church and state, filed a lawsuit Thursday (May 5) in the U.S. District Court for the District of Columbia against the Rev. Patrick Conroy, a House chaplain.

It alleges that Conroy, a Jesuit priest, denied the application of FFRF co-president Dan Barker to deliver a “guest invocation” before a session of the House. The suit claims Conroy rejected Barker, an avowed atheist, because he is “not a minister of the gospel.”

The suit was filed on what is observed by many, including the Congress, as National Day of Prayer.

The suit claims Barker was rejected due to discrimination against atheists. It claims Barker fulfilled all the requirements of a guest chaplain, which include being ordained and addressing a “higher power” in the invocation.

Barker, author of “God: The Most Unpleasant Character in All Fiction” and frequent speaker at atheist and humanist gatherings, was ordained in 1975 by a Christian group in Standard, Calif., and has maintained his ordination despite announcing his atheism in 1984.

Barker planned to address his invocation to the higher power of “We, The people of these United States.”

“We take some satisfaction in filing this lawsuit on the National Day of Prayer, an unconstitutional law enacted at the behest of the Rev. Billy Graham in 1952 requiring the president to issue an annual proclamation exhorting citizens ‘to turn to God in prayer, at churches,'” Barker said in a statement.

The suit also names Speaker of the House Paul Ryan, whose office oversees the Office of the Chaplain in the U.S. House.