Scruffy_Scirocco has written a remarkable piece on the Trinity and on the Christian-Muslim differences around it. You refer in your article to the early church, and to the consensus reached on the Trinity and other dogma. However, it might be useful to note that the early church, like the modern Catholic, Orthodox and Episcopalian/Anglican denominations, believed very clearly in apostolic succession. A timeline could look like this:

1. Christ is incarnated, does his missionary work, assembles his apostles, dies and rises again (one would have to believe all that, of course, in order to accept the rest)

2. The apostles are given a clear mandate and the power to exercise that mandate and to hand it on (Pentecost) -- this is accepted by all the early church fathers (at least, those that accepted it were confirmed in their belief)

3. The councils (Nicea, Calcedon, etc.,) accept some but not all of the books written about Christ, and the Old Testament, as divinely inspired and inerrant, and assemble the bible. The councils are legitimate by virtue of apostolic succession.

4. The combination of scriptural evidence and sacred tradition (supported by apostolic succession) results in numerous doctrinal development and dogmatic reviews and explanations.

The commonly accepted views of Christians concerning the Trinity are, therefore, based on the assumption that apostolic succession is a fact, that the bible and tradition cannot lie -- or, at least, that any lies will be found out and corrected -- and, thus, are to be believed.

This would be a Catholic or Orthodox summary of the events. I don't know that Protestants would agree, their ideas on Apostolic succession being different.

The Muslims have no similar doctrines and, I would imagine, cannot see the point -- though the Shi'a have something fairly close in their insistence on the succession of the Caliphate through Ali.

The Holy Spirit is an interesting concept -- and you are right to raise it in your article. If the Holy Spirit is divine, then surely it is a second person; and, if a second person is allowable, why not a third?