So if Trinity Sunday isn't an interruption to the narrative, what is it?
The Old Testament shows us God the Father. The New Testament shows us Jesus, and Jesus explains (repeatedly) his relationship with God the Father. So far so good.
Then the Holy Spirit shows up.
If we think of Pentecost just as a part of the whole Easter-Ascension-Pentecost thing, it can seem like the Holy Spirit is kind of random. But there's more to Pentecost than just its place in the larger arc of the Easter season. And the Holy Spirit is anything but random.
Making this clear is one of the reasons for Trinity Sunday.
More on this tomorrow.