What a neat idea for your worship service!I used to run the PowerPoint at church, so I have a thought on this. Although it may be possible to create some sort of VBA code that operates like an alarm, here's a more straightforward approach, ASSUMING someone is actively operating the PowerPoint during the service. If so, I would create a separate slide for the "Parent Message" (consider putting it at the end of the deck as a hidden slide). Then use the following technique to allow the PowerPoint operator to easily jump in and out of that slide at any point in the presentation.
1) On the slide master, create a small rectangle set to 99% transparent. Put this rectangle somewhere the operator can easily find since it will be invisible, but I would avoid putting it at the bottom left since a toolbar typically covers that area in Presenter Mode.
2) Select the mostly-transparent box and use the Insert menu to associate it with either a Hyperlink or an Action from the ribbon. In the hyperlink or action window that appears, tell program to hyperlink to the slide containing the "Parent Message"
3) On the parental message slide, select an object (another invisible rectangle, for example) and Insert<Action "Hyperlink to Last Slide Viewed"
Make sure the new rectangle is in a different place than the one on the master slide.