Stephen joined politicians from almost 50 countries to sign an agreement in New York pleging to work together to advance religious freedom.
The Panel of Parliamentarians for Freedom of Religion or Belief (IPP FoRB) was launched last year in response to the rising crisis of religious or belief based persecution, both by terrorist groups and authoritarian governments.
Speaking to Premier Radio during the visit, Stephen said: "We've, all of us signed letters to three governments around the world where persecution is a real problem at the moment. One of them is Iran, another is Vietnam; and the third is Myanmar or Burma.
He added "Experience does suggest that letters like that signed by a wide range of parliamentarians from lots of different countries do have an effect on the governments that receive them."
In Iran only Armenians and Assyrians are allowed to be Christians, and even they are treated as second class citizens. Ethnic Persians are by definition Muslim, and any Muslim who leaves Islam is considered an 'apostate' and therefore 'unclean'. Churches are frequently monitored and raided by secret police; at least 75 Christians were arrested in 2014.
Stephen suggested that some progress had been made on Iran as a result of the presence of an Iranian Ayatollah, who represented the Academy of Sciences in Iran. Stephen said: "I think it rather a remarkable breakthrough to have someone like that at a conference like this and talking good sense about the importance of religious freedom."