A decent, if short read on the topic.
I think at this point it clearly shows its age, in terms of scope (to what degree or how he was Christian), approach (implication Nicean Creed was correct), and terminology (Unconquered Sun used where Sol Invictus is commonly used now).
As this is part of the old 'teach yourself' range of historical non-fiction, the author leaves you to make conclusions rather than really hammer home their position. For what it's worth, I think Christianity was to Constantine the last, ultimate imperial cult. His adolescence was peppered with Christian persecution which only seemed to bring personal difficulty to Augustus' and Caesars. Meanwhile he himself, as well as his father, not indulging in the persecution, seemed charmed by luck. He doesn't seem interested in personal salvation, only imperial protection.
That raises its own question, of what the 'true' Church is; whether Nicean Creed is a bastardisation of an authentic heterogeneous faith, or an act of distillation of authentic religion from heresy. Much like the collection of Qur'ans and their return standardised, with sections from some purged during the process, the faithful have to hope for a miracle in this process, that God was watching over the actions of men and guiding them.
That attempt to make such an imperial cult out of an existing faith would long-term lose the Empire Egypt and the Levant. Once those dissenters were lost, leaving the Empire in faith homogeneous, the religion acted as a redoubt for what was left of the Empire. Constantine may have doomed the Empire in the form he knew it, but fashioned the means for an unconquerable rump out of it. God moves in mysterious ways...