The phenomenon of "spiritual but not religious" people has been widely discussed. I think there's a similar phenomenon, found within the Church: that of "religious but not faithful" people.
As yesterday's post suggests, by "not faithful" I mean literally "not full of faith." I don't mean "not subscribing to this or that list of doctrines." Dissent is a symptom, not a description, of lack of faith. When His disciples cried to Jesus, "We are perishing!," He didn't rebuke them for heresy.
There are at least three causes of a religious (i.e., practicing) Christian having little faith: doubt, pride, and childishness.
Doubt is the ordinary opponent of faith. For whatever reason, a person might simply not have faith in Jesus. Doubt can be limited in scope (e.g. I might doubt that Jesus loves me) or more general (did God really become man?).
Pride resists faith because, well, the proud one just knows better. Perhaps Jesus was God and did die for our sins, but He wasn't really all that advanced, you know. No doubt He did His best, but much of what He said just won't do. And have you looked at the riffraff He left in charge?
Not that the prideful unfaithful necessarily believe the Gospels accurately reflect what Jesus said, or that He really did leave the riffraff in charge in the manner in which the Church teaches He did. To be prideful is to make oneself the judge and teacher of Revelation, to interpret Scripture and Tradition in the light of oneself.
Finally, by childishness I mean a woolyheaded relationship with Christ that hasn't advanced beyond the nursery. The "God" in which such people place their faith is an empty bucket with the words "Jesus Loves Me" painted on it. Whatever is nice, whatever is tolerant, whatever brings the greatest tranquility to the greatest number: this is what those who childishly lack faith put their faith in. They say things like, "The Jesus I believe in would never do something like that," without seeming to think that the Jesus they believe in might not be the Jesus Who is our Savior.
More causes of faithlessness could be named -- fear, for example, and ignorance -- but here my point is that people can be too proud or too childish to even notice that they are not faithful disciples of Christ. The proud correct Jesus' teachings to their own satisfaction, and might then follow that tolerably well, while the childish just invent His teachings out of whole (soft, fluffy) cloth.
This makes it difficult to challenge their lack of faith -- though no one who hasn't sent mountains into the sea should be too confident in his own faithfulness.