Michael Flynn, Donald Trump’s first national security adviser, was widely condemned after calling for the establishment of “one religion” in the US.

Religious freedom is enshrined in the first amendment to the US constitution, which says “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof”.

Regardless, at a rally staged in San Antonio on Saturday by the Christian “nonprofit news media network” American Faith, Flynn said: “If we are going to have one nation under God, which we must, we have to have one religion. One nation under God and one religion under God.”

In response, the Minnesota Democrat Ilhan Omar, one of the first Muslim women elected to Congress, said: “These people hate the US constitution.”

Mark Hertling, a retired general and media commentator, called Flynn, himself a retired general, “an embarrassment to the US army”.

“His words are disgusting,” Hertling said.

On Sunday, the veteran reporter Carl Bernstein told CNN that Flynn, as one of the “knaves and fools and dangerous authoritarian figures” with whom Trump surrounded himself in and out of office, was “saying out loud things that have never been said by an aide or close associates to the president of the United States”.

Bernstein added: “It should be no surprise to know that Michael Flynn is saying the kind of things that he is saying, but what’s most significant here is that much of the Republican party … something like 35% in in exit polls said they favour Trump because Christianity is being taken away from them.

“So Michael Flynn is not that far away from huge numbers of people in this country.”

Flynn is no stranger to controversy. Fired from a senior intelligence role by Barack Obama, he became a close aide to Trump before resigning as national security adviser after less than a month in the role, for lying to the FBI about contacts with Russians.

Flynn pleaded guilty to one criminal charge under Robert Mueller’s investigation of Russian election interference and links between Trump and Moscow, a plea he sought to withdraw before receiving a pardon from Trump.

He has since emerged as an influential figure on the far right, linked to the QAnon conspiracy theory and appearing to advocate armed insurrection.

In San Antonio, Flynn called the indictment of another Trump ally, Steve Bannon, over the investigation of the Capitol attack, an “abuse of freedom of speech” – another first amendment freedom.

The Capitol was attacked on 6 January by Trump supporters seeking to overturn his election defeat. Flynn is himself the subject of a subpoena from the investigating House committee. On Friday, he told Fox News he had nothing to hide.

In Texas, Flynn called the House investigation “a crucifixion of our first amendment freedom to speak, freedom to peacefully assemble”.

His remarks about religion attracted support from a prominent contender in a vicious party fight for a Republican Senate nomination in Ohio.

Josh Mandel, a former Ohio state treasurer, tweeted: “We stand with General Flynn.”

Mandel’s own religion has been the subject of debate and controversy. In September, the Forward published an op-ed which asked if he was “obscuring his Jewishness” in order to appeal to far-right Christian voters.

In response, Mandel described himself as a “Proud American. Proud Jew. Proud Marine. Proud Zionist. Everything Democrats hate.”

Mandel’s religion was the subject of a controversial attack ad from another Republican hopeful, Mark Pukita, who denied charges of antisemitism.

Amid criticism of his support for Flynn, Mandel said “freedom of religion [is not equal to] freedom FROM religion”. He also said: “America was not founded as a secular nation.”