We know Bob killed Jane because he was in her house on the night of the murder.
Dave, Ralph, and Fred were also in the house the night of the murder so you argument is special pleadingIt is taking a line of reasoning that can be applied to many and applying it to one. If you don't believe it to be true for the many then you can't use this argument to prove it is true for the one. That is if you don't believe all four must be guilty of murder then just the presence of a person in that house on that night is not enough evidence. If it is not enough evidence for Dave or Ralph or Fred then it is also not enough evidence for Bob. It is important to note that this does not prove Bob did not kill Jane. He might have. It just means you need some more evidence to show it.
So why are Christians accused of engaging in special pleading? In the US, most Christians are protestants. Even the Catholics are often protestant in their thinking. Protestants engage in special pleading when they choose which parts of Christian tradition to embrace and which parts to reject. Here is GK Chesterton:
Every great heretic had always exhibit three remarkable characteristics in combination. First, he picked out some mystical idea from the Church's bundle or balance of mystical ideas. Second, he used that one mystical idea against all the other mystical ideas. Third (and most singular), he seems generally to have had no notion that his own favourite mystical idea was a mystical idea, at least in the sense of a mysterious or dubious or dogmatic idea. With a queer uncanny innocence, he seems always to have taken this one thing for granted. He assumed it to be unassailable, even when he was using it to assail all sorts of similar things. The most popular and obvious example is the Bible. To an impartial pagan or sceptical observer, it must always seem the strangest story in the world; that men rushing in to wreck a temple, overturning the altar and driving out the priest, found there certain sacred volumes inscribed "Psalms" or "Gospels"; and (instead of throwing them on the fire with the rest) began to use them as infallible oracles rebuking all the other arrangements. If the sacred high altar was all wrong, why were the secondary sacred documents necessarily all right? If the priest had faked his Sacraments, why could he not have faked his Scriptures?So the objection being raised is often valid. Why is the bible legit and the Eucharist and the papacy not legit? The come from the same church?
Special pleading also seems prevalent in biblical interpretation. You read one biblical argument that someone accepts and another one that the same person rejects and often they seem pretty similar. What 1 Cor 11 says about head coverings is not taken that seriously. Why not? Some protestants are OK with divorce but not OK with gay marriage. Both are condemned in scripture. Isn't that special pleading?
I saw this as a protestant. A big issue back then was female ordination. i read biblical arguments in favor and biblical arguments against. There did not seem to be a principled way to determine which to believe. So you just pick one. That is you engage in special pleading.
Catholics get around this problem by accepting all of Christian tradition. There is a consistent way to give weight to certain teachings. Ecumenical councils are very important. Popes matter more than bishops. Doctors matter more than saints. All this data helps us know how to interpret scripture consistently and correctly.
The trouble with many Catholics is they don't do this consistently. They might follow the church on abortion but not on contraception. They might believe in the real presence of Jesus in the Eucharist but not believe anyone can go to hell. Then they are right back to special pleading.
The other way we are accused of special pleading is in choosing to follow Jesus rather than Mohammad or Buddha or Joseph Smith or L Ron Hubbard. It is popular to say,"If you were born in Iran you would be Muslim. You are only Catholic because you parents were." People don't say that to me because I am a convert but many people hear this and it often rings true.
We have to admit we don't come to this world as an outside observer. We are very much in the world. Atheists are in it as well. Many reject the religion of their youth more strongly because they grew up in it and had a bad experience. Yet that does not make all religions the same. They are all some attempt at connecting with God or at least some combination of meaning and morality. The question is not whether there are similarities. The question is whether there is a principled difference.
Once you get past the basics there really are quite massive differences between the religions. Mohammad, in particular, is often asserted to be just like Jesus. Yet you look at the data. He was violent. He gained political power and imposed his will on others. He was a typical tyrant in many ways. If you lived in Galilee at the time of Jesus and in Mecca at the time of Mohammad they would not seem similar at all. Mohammad is more like Pilate than he is like Jesus.
Same with the Koran and the Bible. If you want to assert they are the same then go read them both and see if you still believe that. They are very different books. The New Testament is awe inspiring. The Koran inspires more confusion than anything else.
Of course you would expect false religions. Can you tell a true one from a false one? Often you can. If you can't God won't reject you for it.