In his introduction to Mere Christianity C.S.Lewis says that once you decide to become a Christian it is like coming into the hall of a great house, and off the hall there are lots of side rooms. If becoming a Christian is coming into the great hall, choosing a denomination is deciding which room you want to lodge in.

Now I am a great fan of C.S.Lewis, and I can count on one hand the times I have ever disagreed with the great man, but this is one of them, and it’s a pretty big disagreement. What has long troubled me about Lewis’ hall and side rooms analogy is that he undercuts his own argument. He says that you mustn’t choose the side room according to the furnishings or decoration–in other words, you mustn’t choose a denomination because you like the worship or think the music is nice, or you like the youth pastor or any other subjective matter of opinion. Instead, he says, you must choose according to which one you think is most true.
This reveals an alarming attitude of subjective relativism in the great man. What! Must we choose according to what we think is most true? How shall we make such a choice? What criteria shall we use to determine what is most true? Lewis never says. He never even hints that there are other criteria of authority by which we should choose. He simply says that we must ourselves choose according to which room we think is most true.
The other relativistic part of his teaching is the assumption that all the rooms are of equal value and equal worth, and yet how can this be when he has said we must choose according to which ones are most true? If some are truer than others, then they are not all of the same worth and value. Is their truthfulness (and therefore their value) measured simply by our making the choice? That is to say, “I have chosen this room, therefore this room must be the one that is most true, at least, this room is the one that is most true for me.”
I am no great Lewis scholar, but I know Lewis well enough to know that he would tut tut, snort and roar with dismay at anyone who would propose such a relativistic way of deciding a religious denomination. Indeed, what if the whole analogy were to be put, not to deciding a Christian denomination, but a religion? What if he would have said, “Once you have come to believe in God you have entered the large hall of a great house called Theistic belief. Now you must choose which religion to follow. Here are the Muslims, there are the Hindus, over here we have the Jews, the Bahais are there, the Mormons, the Christians etc. It is up to you to choose the one you think most true. Happy hunting!”
Alas, he would refute such relativism, and rightly so. Why then, did Lewis make such a blunder, if blunder it is? I’m sure he saw the logical mis-step–he was too brilliant not to. Perhaps he was concerned to  be ecumenical and not offend any group of Christians. It is my opinion, however, that there were certain paths Lewis chose not to travel. 
You see, if some of those rooms are more true than others, than by definition, there must be one room which is most true. If there is one room which is most true, how shall we decide which it is? Lewis has said we dare not choose because we like the stained glass, the architecture, the history, the music or the preaching. We must choose according to facts. We must choose according to the Scriptures. We must turn to the writings of the apostles and prophets. We must look to the writings of the early church historians and theologians. 
We must go an a quest to find that one truth which must be big enough to encompass all the others. We must look for that one church from which all the others have sprung. We must look for the one church which is older, more venerable and larger than all the others. I think Lewis knew that road led to Rome, and he decided that was a path he could not follow.