For the curious, the dispute can be reduced to Jesus. Mormons assert that because they believe Jesus is divine, they are Christians by default. Christians respond that because Mormons don’t believe — in accordance with the Nicene Creed promulgated in the fourth century — that Jesus is also the Father and the Holy Spirit, the Jesus that Mormons have in mind is someone else altogether.
We run into a problem right away, because it’s clear from this short summary that Mason has no idea what the Nicene-(Constantinopolitan) Creed says or what it means. Trinitarian orthodoxy set down at Nicaea and Constantinople specifically does not teach that “Jesus is also the Father and the Holy Spirit.” Confusing the three Persons with one another is one of the things that the fathers at these councils were intent on avoiding while still affirming the divinity of the Son and the Holy Spirit. I would go on, but that might be considered “acrimonious niggling.”
Mason seems to have just invented a heretical doctrine that no one in antiquity ever accepted, but which would have been rejected by Nicaea and all of the other ecumenical councils if they had ever encountered it. No wonder he is tired of talking about doctrinal differences. He doesn’t seem terribly interested in getting the details right. This is an unfortunate mistake, because disagreement about core theological doctrines is at the heart of the real differences between Nicene Christianity and Mormonism. It would have been interesting and refreshing to read an argument acknowledging those differences without pretending they are irrelevant.