Of course it's probably not legal to record a conversation without the consent of all parties. It's not here in Australia anyhow.
That's interesting. If it were anyone else, they could file a complaint about you recording them on the sly. But these guys are claiming that they have a public ministry, so you would be recording a public event that they didn't bother to invite anyone else to. They would also be going against scripture, as matters concerning misconduct should first be brought to you by the wronged party, and then be put before the congregation, another public matter. Their unscriptural secret dealings are their undoing in this.