Reply Obj. 4: The Apostles, led by the inward instinct of the Holy Ghost, handed down to the churches certain instructions which they did not put in writing, but which have been ordained, in accordance with the observance of the Church as practiced by the faithful as time went on. Wherefore the Apostle says (2 Thess. 2:14): "Stand fast; and hold the traditions which you have learned, whether by word"—that is by word of mouth—"or by our epistle"—that is by word put into writing. Among these traditions is the worship of Christ's image. Wherefore it is said that Blessed Luke painted the image of Christ, which is in Rome. _______________________
FOURTH ARTICLE [III, Q. 25, Art. 4]
Whether Christ's Cross Should Be Worshipped with the Adoration of Latria?
Objection 1: It would seem that Christ's cross should not be worshiped with the adoration of latria. For no dutiful son honors that which dishonors his father, as the scourge with which he was scourged, or the gibbet on which he was hanged; rather does he abhor it. Now Christ underwent the most shameful death on the cross; according to Wis. 2:20: "Let us condemn Him to a most shameful death." Therefore we should not venerate the cross but rather we should abhor it.
Obj. 2: Further, Christ's humanity is worshiped with the adoration of latria, inasmuch as it is united to the Son of God in Person. But this cannot be said of the cross. Therefore Christ's cross should not be worshiped with the adoration of latria.
Obj. 3: Further, as Christ's cross was the instrument of His passion and death, so were also many other things, for instance, the nails, the crown, the lance; yet to these we do not show the worship of latria. It seems, therefore, that Christ's cross should not be worshiped with the adoration of latria.
On the contrary, We show the worship of latria to that in which we place our hope of salvation. But we place our hope in Christ's cross, for the Church sings:
"Dear Cross, best hope o'er all beside,
That cheers the solemn passion-tide:
Give to the just increase of grace,
Give to each contrite sinner peace."
[*Hymn Vexilla Regis: translation of Father Aylward, O.P.]
Therefore Christ's cross should be worshiped with the adoration of latria.