I'm going to start by being slightly off-topic for answering your question and then swing around to tie your question in. And I'm going to stick to well-known Scriptural principles, so unless someone demands it, I'm going to omit the Scripture quotes that I would normally include, in the interest of keeping this shorter than my usual answer.
A good first question is "Is it sinful to question God?" Since many questions about Christianity are basically questioning the truthfulness of God's word.
Before I get into the rest: An analogy
Driving a car is not illegal. Hitting an unexpected patch of ice, and sliding into another vehicle, killing passengers in either car is unfortunate, but not illegal. (Unless you are drinking, or the accident was caused by breaking other laws like speeding, negligence, cell-phone use while driving, etc.) However, using a car to intentionally kill someone is.
The point of the example above is that driving the vehicle isn't the crime. The vehicle is the tool to commit the crime. Likewise, questions can be tools used to commit sins.
Scripturally, it's quite easy to show that questioning God in and of itself isn't sinful. Job questioned God repeatedly. David questioned Him. Even Jesus questioned God the Father. "Why have you forsaken me?"
However, there are several things that are undeniably considered sins that are acheived via asking questions:
- Justifying your sin, or tempting others to sin by asking "Did God really say that"
- Genesis 3:3: 3 "And he said unto the woman, Yea, hath God said, Ye shall not eat of every tree of the garden?"
- Using questions to mock God. (Your average refute-this atheist question that tries to show us that the whole idea of Christianity is stupid because God is a figment of our imagination, for example."
I'm sure there are more that I could list and think of, but the principle is pretty basic: Questioning God isn't sinful in and of itself, but if your question is being used to deny, defy, or mock God, it is.
Likewise, an honest question seeking an answer, to clarify a point, or even to clarify a point that you understand, in order to help educate others about the Truth of God wouldn't be sinful.
Applied to Christianity as a whole, it's easy to see, as there are dozens of examples even on this site, it's easy to ask questions about Christianity where the sole purpose is to mock God, or deny/defy Him. In that context, asking questions and answers about Christianity in general can be sinful. But questions and answers that are honestly seeking would not be.