On the subject of the Trinity: Why did the first century christians... ?

Good question. Since the early Christians did not believe in it what makes you say that it is the truth? I think that so much has happened throughout the centuries that we really have to let our hearts and minds lead us to the truth. I think that Paul did not spend a lot of time on it because it wasn't a problem for him. He knew that God sent down Jesus and gave everyone who believes the gift of the Holy Spirit and that all three are related/interconnected yet mutually exclusive. The blasphemy of the Holy Spirit is very interesting and frustrating because Jesus does seem to separate himself BUT everything that I have read about it says it is the rejection of the Holy Spirit, i.e. the rejection of accepting Christ. I think that the first Christians wouldn't recognize modern day Christians but as time pes things and ideas change for different reasons. I know that this isn't a direct answer but I don't have one. Good luck.