Augustine on honoring martyrs
In his dialogue with Faustus regarding giving honor to (dead) martyrs, Augustine wrote (English translation from Nicene and Post Nicene Fathers or NPNF, Series I, Vol. 4 with underlined emphasis added, NPNF is available online at www.ccel.org):
It is true that Christians pay religious honor to the memory of the martyrs, both to excite us to imitate them and to obtain a share in their merits, and the assistance of their prayers. But we build altars not to any martyr, but to the God of martyrs, although it is to the memory of the martyrs. No one officiating at the altar in the saints’ burying-place ever says, We bring an offering to thee, O Peter! Or O Paul! Or O Cyprian! The offering is made to God, who gave the crown of martyrdom, while it is in memory of those thus crowned. The emotion is increased by the associations of the place, and love is excited both towards those who are our examples, and towards Him by whose help we may follow such examples. We regard the martyrs with the same affectionate intimacy that we feel towards holy men of God in this life, when we know that their hearts are prepared to endure the same suffering for the truth of the gospel. There is more devotion in our feeling towards the martyrs, because we know that their conflict is over; and we can speak with greater confidence in praise of those already victors in heaven, than of those still combating here. What is properly divine worship, which the Greeks call latria [Greek latreia], and for which there is no word in Latin, both in doctrine and in practice, we give only to God. To this worship belongs the offering of sacrifices; as we see in the word idolatry, which means the giving of this worship to idols. Accordingly we never offer, or require any one to offer, sacrifice to a martyr, or to a holy soul, or to any angel. Any one falling into this error is instructed by doctrine, either in the way of correction or of caution. For holy beings themselves, whether saints or angels, refuse to accept what they know to be due to God alone. We see this in Paul and Barnabas, when the men of Lycaonia wished to sacrifice to them as gods, on account of the miracles they performed. They rent their clothes, and restrained the people, crying out to them, and persuading them that they were not gods. We see it also in the angels, as we read in the Apocalypse that an angel would not allow himself to be worshipped, and said to his worshipper, “I am thy fellow-servant, and of thy brethren.”
Augustine, Against Faustus 20.21
Augustine defended Christian practise of giving honor to (dead) martyrs and insisted it was not idolatry. Note that he believed that those martyrs in heaven are able to assist us in prayer. Augustine used the (Greek) word latreia (Latinized into latria) to indicate worship to God and to Him alone, a term that is still being used by the Catholic Church today. In his other and well known work, City of God, he differentiated between latria and dulia, the latter indicates honor or veneration given to saints (English translation from NPNF Series I Vol. 2):
For this is the worship which is due to the Divinity, or, to speak more accurately, to the Deity; and, to express this worship in a single word as there does not occur to me any Latin term sufficiently exact, I shall avail myself, whenever necessary, of a Greek word. Latreia, whenever it occurs in Scripture, is rendered by the word service. But that service which is due to men, and in reference to which the apostle writes that servants must be subject to their own masters [Ephesians 6:5], is usually designated by another word in Greek [douleia] whereas the service which is paid to God alone by worship, is always, or almost always, called Latreia, in the usage of those who wrote from the divine oracles.
Augustine, City of God 10.1