The phrase "in the name of the Father, and of the Son, and of the Holy Spirit" (original Greek εἰς τὸ ὄνομα τοῦ Πατρὸς καὶ τοῦ Υἱοῦ καὶ τοῦ Ἁγίου Πνεύματος, Latin in nomine Patris et Filii et Spiritus Sancti), or words to that effect.
It is taken from the words of Jesus of Nazareth in Matthew 28:19, and used in many Christian liturgical traditions, particularly in baptism and in the sign of the cross.