Here is St. Robert Bellarmine's Sermon on the actual cross that Jesus was crucified on.
But before I begin to write on the words which our Lord spoke from the Cross, it seems proper that I should say something of the Cross itself, which was the pulpit of the Preacher, the altar of the Sacrificing Priest, the arena of the Combatant, the workshop of the Wonder-worker. The ancients commonly agree in saying that the Cross was made of three pieces of wood; one upright, along which the body of the crucified person was stretched; another transverse, to which the hands were fastened; and the third was attached to the lower part of the cross, on which the feet of the condemned rested, but fastened by nails to prevent their moving about. The ancient Fathers of the Church agree in this opinion, as St. Justin and St. Irenaeus. These authors, moreover, clearly indicate that each foot rested on the foot-board, and that one foot was not placed over the other.
Hence, it follows that Christ was nailed to the Cross with four nails, and not with three, as many imagine, who in pictures represent Christ, our Lord, as nailed to the Cross with one foot over the other. Gregory of Tours, distinctly says the contrary, and confirms his view by an appeal to ancient pictures. I, for my part, have seen in the Royal Library at Paris, some very ancient manuscripts of the Gospels, which contained many pictures of Christ crucified, and these all had the four nails.
St. Augustine, and St. Gregory of Nyssa, say that the upright piece of the Cross projected a little from the transverse piece. It would seem that the Apostle also insinuates the same, for in his Epistle to the Ephesians St. Paul writes: "That you may be able to comprehend with all the saints, what is the breadth, and length, and height, and depth." This is clearly a description of the figure of the Cross, which has four extremes; breadth in the transverse piece; length in the upright piece; height in that part of the Cross which stood out and projected from the transverse part; and depth in the part which was buried in the earth.
Our Lord did not endure the torments of the Cross by chance, or unwillingly, since He had chosen this kind of death from all eternity, as St. Augustine teaches from the testimony of the Apostle: "Jesus of Nazareth being delivered up, by the determinate counsel and fore-knowledge of God, you by the hands of wicked men have crucified and slain." And so Christ, at the beginning of His preaching, said to Nicodemus: "As Moses lifted up the serpent in the desert, so must the Son of Man be lifted up, that whosoever believes in Him may not perish, but may have life everlasting." He often spoke to His Apostles about His Cross, and encouraged them to imitate Him by the words: "If any man will come after Me, let him deny himself, and take up his cross and follow Me."
Our Lord alone knows the reason that induced Him to choose this manner of death. The holy Fathers, however, have thought of some mystical reasons, and have left them to us in their writings. St. Irenaeus, in the work of his to which we have referred, says that the words, "JESUS OF NAZARETH, KING OF THE JEWS," were written over that part of the Cross where the two arms meet, to give us to understand, that the two nations, of Jew and Gentile, which had up to that time been estranged from each other, were henceforth to be united into one body under the one Head, Christ.
The Cross is the Key To Heaven
St. Gregory of Nyssa, in his sermon on the Resurrection, says that the part of the Cross which looked towards heaven, shows that heaven is to be opened by the Cross as by a key; that the part which was buried in the earth shows that hell was despoiled by Christ when He descended thither; and that the two arms of the Cross, which stretched towards the cast and west, show the regeneration of the whole world by the Blood of Christ.
St. Jerome, on the Epistle to the Ephesians, St. Augustine, in his Epistle to Honoratus, St. Bernard, in the fifth book of his work on "Consideration," teach that the principal mystery of the Cross was briefly touched upon by the Apostle in the words: "What is the breadth, and length, and height, and depth." The primary signification of these words points to the attributes of God; the height signifies His power; the depth, His wisdom; the breadth, His goodness; the length, His eternity. They have reference also to the virtues of Christ in His Passion; the breadth, His charity; the length, His patience; the height, His obedience; the depth, His humility. They signify, moreover, the virtues which are necessary for those who are saved through Christ. The depth of the Cross means faith; the height, hope; the breadth, charity; the length, perseverance. From this we gather that only charity, the queen of virtues, finds a place everywhere, in God, in Christ, and in ourselves. Of the other virtues, some are proper to God, others to Christ, and others to us. Consequently it is not wonderful that in His last words from the Cross, which we are now going to explain, Christ should give the first place to words of charity.