Belfast Pastor James McConnell, who is facing prosecution because of remarks he made branding Islam "satanic", has won the backing of a Muslim imam and a Roman Catholic priest.

Dr Muhammed Al-Husseini, a Senior Fellow in Islamic Studies at the Westminster Institute, wrote in an article for the Belfast Telegraph that he was prepared to go to jail with McConnell if he was convicted. A parish priest in Crossgar, Rev Patrick McCafferty, described the threatened prosecution as "an absolute absurdity", saying: "Pastor McConnell has very strong views, but are they going to go now every Sunday to every church, every gospel hall and monitor over things?"

In a sermon delivered during the Meriam Ibrahim case in Sudan and streamed on the internet, McConnell said: "Islam is heathen, Islam is satanic, Islam is a doctrine spawned in hell." He was offered an "informed warning" under the 2003 Communications Act but refused to accept it. Northern Ireland's Public Prosecution Service said that the offence was "sending, or causing to be sent, by means of a public electronic communications network, a message or other matter that was grossly offensive".

Al-Husseini said that he "strongly upholds the moral right" for people of all faiths to debate. "Against the flaming backdrop of torched Christian churches, bloody executions and massacres of faith minorities in the Middle East and elsewhere, it is therefore a matter of utmost concern that, in this country, we discharge our common duty steadfastly to defend the freedom of citizens to discuss, debate and critique religious ideas and beliefs – restricting only speech which incites to physical violence against others."

He added that "in a free and democratic society, we enter into severe peril when we start to confuse what we perhaps ought or ought not to say, with what in law we are allowed to, or not allowed to say.

"While those of us who hold clerical office as Christian pastors and priests, Jewish rabbis or Muslim imams, should rightly have due care and regard to the leadership role we exercise when we make public speeches, nevertheless our foremost duty remains to express theological ideas in good conscience before God."

He expressed his "deep concern and opposition to the criminalising of theological disagreement, at a time when our society should in fact be fostering better quality disagreement". In that spirit, he said, "I further undertake that if Pastor McConnell is convicted and sent to prison, I shall go to prison with him."