The United Church of God denies the doctrine of the trinity primarily by arguing that the Holy Spirit is not a Third Person, but a power.

I'd say their more fundamental error is believing that the Father and Son "are two separate Beings that comprise the God Family." UCG thinks one day there will be many more than just two.

That's straight-up pagan polytheism, which goes against everything traditional Catholicism stands for.

It's chief among the reasons I insist UCG and others like the cannot be properly call "Christian."