I am one who brings the question of "what is a Christian?". "Many will come to me saying 'look at all the good deeds I did in your name' and to them I will say plainly 'I never knew you, and you are a worker of iniquity'" (
Matthew 7:21-23). There is
John 10:16, that shows that the ones belonging to Jesus Christ are also found outside of the constraints of the religion that was attributed to Him.
The parable of the final judgment in
Matthew 25:31-46 also tells us that the judgment is not based on what we have believed, but how we have treated those who were coming in His spirit: "what you did to even the least of these brothers and sisters of mine, you were doing to me".
He even says that He will draft them from all the nations (which means to say that within all nationalities - whether it be an identity of kin as Orthodox Christian, Catholic, Jewish or Protestant - they are all nations in His eyes. Some translations don't even call them nations, but "peoples").
Therefore it is not the religion that saves, but the fruits of it, and when one is mature, he realises that The Holy Spirit is working with His people through the confines of the religious and social structures that they have been born and formed within (
Ecclesiastes 12:11). Still, the knowledge of Jesus Christ is a dividing matter that defines those who are of His kin (
Matthew 10:34,
1 Peter 2:6,
John 13:35), and everybody has to take a side on the matter when it comes to them, regardless of their religious background. That's where the Qur'an proves that it really is the enemy of Christianity, because it forces its believers to oppose the scriptures that have been given by The Holy Spirit of God and even introduces wild facts that are at odds with the true history of things.
That is true (eg:
Mark 7:13), and it is really the same problem that we are discussing, where Judaism was the religious mechanism for the captivity rather than Islam (
Ephesians 4:8). We see examples in history and the world around us, that the same thief operates within the confines of the Christian religion, as is warned by
Hosea 4:6 and foretold by
2 Thessalonians 2:3 and witnessed by
2 Peter 2:1-2 etc.