How can I explain the Trinity to a Oneness Pentecostal?
The short answer
In Matthew 28:19 Jesus explicitly commands baptism in the name of the Father, Son and Holy Spirit — the source of the Trinitarian formula, not a pope's idea. Acts 2:38 isn't giving a liturgical formula but distinguishing Christ's baptism from John's. Early Church practice was thoroughly Trinitarian.
Jesus' command
In Matthew 28:19 Jesus explicitly tells the apostles to baptize in the name of the Trinity. That is where the Church got the formula; it wasn't a pope's invention.
Acts 2:38
Acts 2:38 is not a liturgical formula. Luke is distinguishing Christ's baptism from John the Baptist's.
The early Church
The practice of the early Church was thoroughly Trinitarian.