V. The transference of the cross from the Lord to Simon of Cyrene signifies the participation of the Gentiles in His sufferings
But as the multitudes went with Jesus to the place of punishment, a certain Simon of Cyrene was found on whom to lay the wood of the cross instead of the Lord; that even by this act might be pre-signified the Gentiles' faith, to whom the cross of Christ was to be not shame but glory. It was not accidental, therefore, but symbolic and mystical, that while the Jews were raging against Christ, a foreigner was found to share His sufferings, as the Apostle says, “if we suffer with Him, we shall also reign with Him”; so that no Hebrew nor Israelite, but a stranger, was substituted for the Saviour in His most holy degradation.
For by this transference the propitiation of the spotless Lamb and the fulfilment of all mysteries passed from the circumcision to the uncircumcision, from the sons according to the flesh to the sons according to the spirit: since as the Apostle says, “Christ our Passover is sacrificed for us,” Who offering Himself to the Father a new and true sacrifice of reconciliation, was crucified not in the temple, whose worship was now at an end, and not within the confines of the city which for its sin was doomed to be destroyed, but outside, “without the camp,” that, on the cessation of the old symbolic victims, a new Victim might be placed on a new altar, and the cross of Christ might be the altar not of the temple but of the world.
Source: Sermons (New Advent)