53 As Bellarmine notes with acumen and accuracy, this appellation of the Body of Christ is not to be explained solely by the fact that Christ must be called the Head of His Mystical Body, but also by the fact that He so sustains the Church, and so in a certain sense lives in the Church, that she is, as it were, another Christ. The Doctor of the Gentiles, in his letter to the Corinthians, affirms this when, without further qualification, he calls the Church "Christ," following no doubt the example of his Master who called out to him from on high when he was attacking the Church: "Saul, Saul, why persecutest thou me?" Indeed, if we are to believe Gregory of Nyssa, the Church is often called simply "Christ" by the Apostle; and you are familiar Venerable Brethren, with that phrase of Augustine: "Christ preaches Christ."
Source: Mystici Corporis Christi (Vatican.va)