The Son, Jesus Christ as redeemer of the world. And the Spirit, the abiding presence of God with us. This Trinitarian understanding of God would take shape in the creeds that followed the tri-fold nature by professing that we believe in God the Father, Almighty and in Jesus Christ his only Son and in the Holy Spirit. If you look at the creeds of the early church, they follow this trinitarian model. The same was often true in hymnody.
Many hymns contain four stanzas.
The first stanza is in praise of God, the second in praise of Christ, the third referring to the Holy Spirit and the final verse summing it up in the belief in the trinity. All this is to sing praises of the one God known in three persons. Paul even wrote of this in the classic benediction to his second letter to the Corinthian Church. That is to say, may you experience the blessing of God's presence in all of God's forms, mysterious as that may be. It reminds me of the priest who was making rounds in a hospital.