The rabbi is often one of the more expert of priestly persons who undergo years of training. He becomes erudite and able to be wise about many things that are spiritual and also about the world. This is a thing which makes him a preferred expert for those kinds of ceremonies that are relatable.
For Christians of course the faith of the rabbi is something they might be quite familiar with. Although the Jewish interfaith rabbi may have been trained in esoteric and exclusive processes for synagogues, nowadays his church and elders could allow him to perform ritual for non Jews. Some best ceremonies are interfaith these days.
For instance, once before persons of different religions could not often wed. And they would find it hard to find sympathizers among their co religionists. These were often set against the love that may occur between a Christian and a Jew for instance and for many it became a tragic thing because these folks would not marry anybody else.
So the fact is that the opening up has become a need and the expectations of many these days is about being able to be free to love anybody. And this model of interfaith or ecumenism practice is going to be the future of all religions, especially the major ones, although it may take years to become a total or overall process.
The Dalai Lama first presented the idea of unifying all major religions of the world. The one faith was like the United Nations of the divine. And this meant that persecution would no longer be something that anyone should fear in this line.
So there are many processes that are being tried now. Even interfaith is an older system that works mostly for Americans although it is adaptable and many in Europe simply will not marry in their faiths but in the temporal or secular systems. The judges, the government officials could make their marriages formal and thus free or any taint of faith.
Religions have been strongly against marriages of persons of differing faith. This is a very old practice that once was the most strictly enforced for members of Judaic system. And they were also very strict against the Christians and Christians reciprocated in the most negative ways.
The thing about the continuity between these two was ignored. Even with the Bible containing all the most important precepts of Judaism, many in the Christian world favored the apartness of Jews. These days this no longer works, and lots and lots of people are getting to demand the kind of thing that works for all those with faith in the divine.
Thus it is now easier to find another person anywhere without fear of not being accepted by their families. The openness extends to other kinds of beliefs too and these days if there is no approval, two people in love will simply go out of their families and have a judge give them a certificate. There is also Las Vegas, and despite its sympathy with Christianity, places there will marry all who want to.