In short, it sucks. It sucks eggs. Really. It's like believing the Gospel, but not having any grace to go with it.
The biggest problem with "losing one's faith" is that your moral compass remains unchanged. Unlike what I suspect many can do - namely ignore their own internal compass - priests are, by and large, a self-selected lot. People who are attracted to the priestly vocations have a strong sense of right and wrong, and at least in my case, I have a physical reaction to it. At the risk of having too graphic an image, when I act in a way that is contrary to my internal sense of ethics (call it the opposite of integrity), I feel that pain in my stomach that is felt when you need to throw up, but can't. My heart pounds, my stomach turns, and I feel ill. Stressed seems so quaint a term. Trust me, its physical.
Jeremiah 20:9 has the prophet telling God:
But if I say, "I will not mention him or speak any more in his name," his word is in my heart like a fire, a fire shut up in my bones. I am weary of holding it in; indeed, I cannot.
For many of us, real honesty is fundamental to that integrity. We cannot preach that which we do not believe, lest that feeling arise. Were losing that honesty and commitment to transparency and integrity something that was lost along with one's belief, it would be easy. We would merely be actors playing a part. Being a charlatan or a hypocrite would have no hold on us. But no, sadly, if its your job to preach and minister, you still have to do it.
I would suggest that there are three categories of "losing one's faith," and there are different effects of each. Hebrews 11:6 says,
"Who ever would come to God must
believe that he exists and
believe he rewards those who do good."
Using that verse as an outline, I would suggest that as Tolstoy said, "Happy [Christians] are all happy alike. But miserable [Christians] are each miserable in their own way."
Category 1a would be what is often called "the long, dark night of the soul."
Many Christian leaders, from Mother Teresa to Billy Graham echo the sentiment of the Middle Ages mystic who describe a period in which God seems to simply be silent. In these cases, the work of ministry is merely a drudge, devoid of the music that God brings. I would argue this is the least "bad" of the categories of losing one's faith. No one suggests that God is anything but silent. Your moral compass remains, your theology is in tact, and your ministry can continue as leader and teacher. Individually, it just becomes a matter of work like anything else. You do what you need to do, you don't violate your conscience, and you just eagerly anticipate God's return, both in your life and the next.
Category 1b would be the full on rejection of God's existence.
NPR had an amazingly good story about a Methodist minister who became an atheist.. In that story, she describes the same gut-wrenching feeling that I describe, and speaks of the consequences in her life. She describes how she came to the rationalization that God is a myth. (If you will forgive the commentary, it seems a natural extension of too cereberal and metaphorical a faith. It can be a very natural progression from mythic to myth.) In her case, she got up and addressed a convention of atheists, much to the applause of that body, but was then somehow astonished that her temporal boss - the Methodist bishop, fired her. Grant you, if I spoke glowingly about a competitors product at the competitor's function and urged everyone to ditch my own company in favor of the competition, I would expect to be fired too, but I digress. The point is in that instance, she was able to find both solace and connection in a new group of "believers" (albeit atheist believers), and restore her sense of integrity. For her, "losing her faith" was simply a job change, and in this day and age, job changes are easy.
In my case as a Baptist minister who found himself desiring a stronger liturgy, changing from the Baptist denomination to the Episcopal one was fairly easy. I could preach as I wanted, and I could continue to minister. Different clothes but the same God. It doesn't turn your world upside down to modify your faith. It's different when it breaks.
Category 2 is the one that sucks the most. It says, "God exists, but he is not good."
That is my current dilemma. I won't bore you with the details, but suffice it to say that the circumstances of my life have made it increasingly difficult to believe that God is good. I can only hold on secure in the knowledge that I am not yet ready to "curse God and die," but I'm beginning to like Job's wife a lot more than Job.
You see, it's real work to maintain a love for a God whom you secretly hate. A double-minded man is unstable in all his ways. When a minister loses his faith, the job is the easy part. Pastors, of course, tend to find alternative employment (I know of one who, after leaving his wife - my college friend and a girl I considered marrying! - became a used car salesman. Talk about about transferrable skills!). They do this because they cannot be honest with themselves in ministry.
No, the hard part isn't the money but the music. You see, for me, God's love is like music. I can live without it, but life lacks color. It lacks joy. I know in turning my back on Him, I turn my back on myself too. I lack purpose, I lack ... music. I am left only with that gnawing pit in my stomach: Insufficent to expurgate, but sufficient to cause distress. Were I able to deny the existence of God, I could rewrite my life. Absent that, I left in a state of limbo that churns against my very core.
In my case, I console myself with the last lines of "Come, Thou Fount of Every Blessing." The hymn writer himself was once a pastor, but also gave it up to, in his words, "chase after worldly things." He so eloquently phrases it:
Prone to wander, Lord I feel it - Prone to leave the God I love. Here's my heart, O take and seal it, seal it to thy courts above!
If only, I think. If only. You can imagine how much it sucks to know the music, and yet be bereft of the assurance that it will come to pass. I personally tend to "blow hot and cold," preaching to myself (really the main I reason I preach!) the goodness of God, hoping only that I can convince myself of that fact. If only I were a better preacher :)
So that, my friend, is what it means to lose one's faith. It is to choose a personal hell in which you get hit on both sides. You become your own worst enemy, and you cannot rest.
If you will forgive the Ferris Beueller riff, "I most heartily do not recommend it."