The road there, if you'll let a guide direct you Who only has at heart your getting lost, (8-9)
Most spiritual leaders are trying to save you, to keep you from getting lost. Think about the famous lines from Amazing Grace: "I once was lost, but now am found." Are you really going to trust someone who says up front he's going to mess with you? That what he really wants is to get you as lost as humanly possible? And yet, there's a real temptation to submit, to allow yourself that escape, isn't there?
Nor need you mind the serial ordeal Of being watched from forty cellar holes (20-21)
This may not be a dragon battle, but in any quest tale or hero's journey (especially those with a holy flavor), you always have a series of obstacles or trials. Here Frost gives a quick nod to these with a "serial ordeal." As long as you disregard those eyes, you'll have passed your first hurdle. You'll be well on your way on your Grail Quest.
And if you're lost enough to find yourself By now, pull in your ladder road behind you And put a sign up CLOSED to all but me. (36-38)
Remember—the Grail is only available to those who are truly worthy. So it would make sense to keep this journey an exclusive one. After all, not everyone is ready for spiritual salvation. You've gotta work for it, says our speaker.
Your destination and your destiny's A brook that was the water of the house, Cold as a spring as yet so near its source, (49-51)
Congrats, Shmoopers. You've made it back to the beginning, the source. What is this source? Well that depends on how you interpret the poem. But in any case, it's clear that these waters are a metaphorical destiny—a place of sustenance and renewal.
A broken drinking goblet like the Grail Under a spell so the wrong ones can't find it, So can't get saved, as Saint Mark says they mustn't. (57-59)
It's all here, folks: the drinking goblet like the Grail, which was the vessel Jesus Christ drank from at the Last Supper, is under a spell so only the chosen ones can find it and be saved, and the unworthy cannot. If you're looking for the hero's journey, this is it: paydirt.
Here are your waters and your watering place. Drink and be whole again beyond confusion. (61-62)
If this doesn't sound like the end of a rousing and inspiring sermon, what does? After the long and winding road, the rocky path, the climb, the serial ordeal, the many thresholds necessary for you to cross in your escape, you've finally arrived at your waters and your watering place.