In his reflection for tomorrow’s Sunday readings, pontifical household preacher Fr. Raniero Cantalamessa links the Feast of the Exaltation of the Cross with the creedal hymn in St. Paul’s Letter to the Philippians:
Let us first say something about the origin of this feast. It recalls two events, distant from each other in time. The first is Constantine’s founding in 325 of two basilicas, one at the site of Golgotha and one over Christ’s sepulcher. The other event, in 628, is the Christians victory over the Persians, which led to the recovery of relics of the cross and their triumphal return to Jerusalem. With the passing of time, however, the feast came to take on a new meaning. It became a joyous celebration of the mystery of the cross, which Christ transformed from an instrument of shame and judgment to an instrument of salvation.
The readings reflect the latter significance of the feast. The second reading contains the celebrated hymn from St. Paul’s Letter to the Philippians in which the cross is seen as the cause of Christ’s “exaltation”: “He emptied himself, taking the form of a slave, coming in human likeness; and found human in appearance, he humbled himself, becoming obedient to death, even death on a cross. Because of this, God greatly exalted him and bestowed on him the name that is above every name, that at the name of Jesus every knee should bend, of those in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father.” The Gospel too speaks of the cross as a moment in which the Son of Man is lifted up “so that everyone who believes in him might not perish but might have eternal life.”
This letter dates to the mid-fifties, the passage cited above is believed to be St. Paul’s recitation of an early liturgical hymn on the divinity of Christ. You might consider it a “proof text”; despite what you may have heard from modernist scholars, belief in Christ’s divinity wasn’t imposed on the faithful in later centuries by the Church’s hierarchy.