The following comes fromt the CNA:
The sign of the cross reminds us of the Trinity which resides in us, of God's name and of our commitment to the faith from the moment of baptism, said Pope Benedict before Sunday's Angelus. He also noted the essential role of the priest in bringing the “Spirit of Truth” to us.
The first Sunday of ordinary time brought with it a return to the Angelus prayer from the traditional Regina Coeli prayer of the Easter season.
With the arrival of "ordinary time," the Holy Father said, the Christian commitment should not decrease. Rather, "entered into the divine life through the sacraments, we are called daily to be open to the action of Grace, to progress in love towards God and neighbor."
Turning to Trinity Sunday, which "recapitulates" God's revelation concerning the Father, Son and Holy Spirit in the Paschal mysteries, Pope Benedict said that despite the inadequacy of human thought and language to explain the One and Triune God, Church Fathers sought to illustrate His mysteries through their lives and deep faith.
"The Trinity, in fact, finds residence in us the day of our baptism," said the Pope, highlighting the priest's words, "I baptize you in the name of the Father, the Son and the Holy Spirit."
And, he added, we are reminded of God's name, in which we are baptized, every time we make the sign of the cross.
Pope Benedict XVI cited the words of theologian Romano Guardini who taught that the sign of the cross puts us "spiritually in order" both before prayer and afterwards "so that what God has donated to us remains in us."
"In the sign of the cross and the name of the living God the announcement that generates the faith and inspires prayer is therefore contained," he emphasized.
Commenting on Jesus' promise to the Apostles that "when he comes, the Spirit of the truth, he will lead you to every truth," the Holy Father pointed out that this takes place in the Sunday Liturgy "when the priests distribute, week to week, the bread of the Word and of the Eucharist."
As St. Jean Vianney observed, continued the Pope, it is the priest who welcomes the human soul to the world, strengthens it and prepares it for its return to God.
The Holy Father closed his pre-Angelus words by inviting the faithful to recite the prayer of St. Hilary of Poitiers for loyalty to the faith which is professed on the day of our baptism and invoking the protection of the Virgin Mary, "the first creature fully inhabited by the Most Holy Trinity," for our continued our pilgrimage on earth.