"AS stated above (a.3), honor or reverence is due to a rational creature only; while to an insensible creature, no honor or reverence is due save by reason of a rational nature. And this in two ways. First, inasmuch as it represents a rational nature: secondly, inasmuch as it is united to it in any way whatsoever. In the first way men are wont to venerate the king's image; in the second way, his robe. And both are venerated by men with the same veneration as they show to the king. If, therefore, we speak of the cross itself on which Christ was crucified, it is to be venerated by us in both ways – namely, in one way in so far as it represents to us the figure of Christ extended thereon; in the other way, from its contact with the limbs of Christ, and from its being saturated with His blood. Wherefore in each way it is worshiped with the same adoration as Christ, viz. the adoration of "latria." And for this reason also we speak to the cross and pray to it, as to the Crucified Himself. But if we speak of the effigy of Christ's cross in any other material whatever – for instance, in stone or wood, silver or gold - thus we venerate the cross merely as Christ's image, which we worship with the adoration of "latria," as stated above (a.3)."
~St. Thomas Aquinas: Summa Theologica, III, q. 25, a. 4.
Glorification of the Cross, by Adam Elsheimer.
Oil on copper, c. 1605; Städelsches Kunstinstitut, Frankfurt.
"Dulia (Greek doulia; Latin servitus), a theological term signifying the honour paid to the saints, while latria means worship given to God alone, and hyperdulia the veneration offered to the Blessed Virgin Mary. St. Augustine (City of God X.2) distinguishes two kinds of servitus: "one which is due to men . . . which in Greek is called dulia; the other, latria, which is the service pertaining to the worship of God". St. Thomas (II-II:103:3) bases the distinction on the difference between God's supreme dominion and that which one man may exercise over another. Catholic theologians insist that the difference is one of kind and not merely of degree; dulia and latria being as far apart as are the creature and the Creator. Leibniz, though a Protestant, recognizes the "discrimen infinitum atque immensum between the honour which is due to God and that which is shown to the saints, the one being called by theologians, after Augustine's example, latria, the other dulia"; and he further declares that this difference should "not only be inculcated in the minds of hearers and learners, but should also be manifested as far as possible by outward signs" (Syst. theol., p. 184). A further distinction is made between dulia in the absolute sense, the honour paid to persons, and dulia in the relative sense, the honour paid to inanimate objects, such as images and relics. With regard to the saints, dulia includes veneration and invocation; the former being the honour paid directly to them, the latter having primarily in view the petitioner's advantage." ~Catholic Encyclopedia
