The Solemnity of the Most Holy Trinity:
Psalm 8:4-5, 6-7, 8-9
First, God the Father is represented our first reading from Proverbs. The imagery would seem to tell us that God knew us before creation itself. While to Christian ears this reading seems to speak of the Son of Man (and is actually a favorite of early Christians), the author is actually referring to humanity. This text predates Christ by hundreds of years, with some of it being attributed to Solomon himself. The New American Bible’s introduction to Proverbs states that “The teaching of the entire book is placed on a firm religious foundation by the principle that "the fear of the LORD is the beginning of knowledge." This idea would seem to be supported by establishing the “omniscience” of God.
Second we see God the Son in our reading from Paul’s letter to the Romans. Here Paul teaches us that it is our faith in Jesus that connects us to God the Father, and through that, a connection with the Holy Spirit.
Finally we see God the Holy Spirit as we read John’s Gospel. Taken from the farewell discourse (prior to the Passion), Jesus is seen teaching the Apostles about the Holy Spirit.
This trifecta of readings not only lets us see the individual characters of God the Father, Son, and Holy Spirit, but also shows us the interdependence of each through the other.
You may also want to check out these supplemental readings:
Scripture from Scratch
Celebrating Mary: Feasts of Our Lady (for May being the month of Mary)