Contact the UP inquirer in the class and ask how he feels about the UP pastor showing up. He/she is likely to feel intimidated and threatened. Get a sense of what interpersonal dynamics may underlie this particular situation; surely this is not the first person ever to leave this pastor’s congregation. Is there hanky-panky? Is this person a major contributor? Is this person a valued leader?
Then call (or better, visit the pastor on his own turf) and tell him your inquirer’s feelings. Explain that RCIA is for serious inquirers to learn about the faith, and not an appropriate venue for his rescue operation; he can do that on his own time. Ask him how he would feel if you came to his inquirers class to reclaim a fallen-away Catholic.
Ask him what he thinks he has to fear. If the Catholic Church is so patently the Synagogue of Satan, then the inquirer he is so eager to save will see it for what it is and come running home. See how the temperature rises or falls. Be careful with taking the conversation further because he can turn it around on you.
If you allow him to attend at this time, and he becomes distruptive, you are setting yourself up for a headline: Catholic Church Forcibly Ejects UP Pastor! He may be deliberately setting you up for just such a situation.
If he seems decent, you might explain that if he wishes to attend the class next year on the same terms as anybody else – merely as an respectful inquirer – he may do so provided he is courteous and does not dominate nor disrupt the class.
Massage his ego. Talk to him man-to-man, Christian-to-Christian, fellow clergyman-to-fellow clergyman. If you visit him in person, take him a gift that won’t threaten him – a *Catechism – *and perhaps a Catholic devotional book on the Holy Spirit, since these guys are Pentecostal, so even if they don’t believe in the Trinity, they do believe in the Holy Spirit . . . If you have a souvenir from the Holy Land, that might be a little bridge maker.