The great True Catholic historian, Philip Schaff, points out as regards the beginning of prayers to the saints in the post-Nicene Church:
Philip Schaff, History of the Christian Church (Volume III, Nicene and Post-Nicene Christianity A.D. 311-390, P. 409-427).
In the first three centuries the veneration of the martyrs in general restricted itself to the thankful remembrance of their virtues and the celebration of the day of their death as the day of their heavenly birth. This celebration usually took place at their graves.
So the church of Smyrna annually commemorated its bishop Polycarp, and valued his bones more than gold and gems, though with the express distinction:
“Christ we worship as the Son of God; the martyrs we love and honor as disciples and successors of the Lord, on account of their insurpassable love to their King and Master, as also, we wish to be their companions and fellow disciples.” Here we find this veneration as yet in its innocent simplicity.
But in the Nicene age it advanced to a formal invocation of the saints as our patrons (patroni) and intercessors (intercessores, mediatores) before the throne of grace, and degenerated into a form of refined polytheism and idolatry. The saints came into the place of the demigods, Penates and Lares, the patrons of the domestic hearth and of the country.
As once temples and altars to the heroes, so now churches and chapels came to be built over the graves of the martyrs, and consecrated to their names (or more precisely to God through them). People laid in them, as they used to do in the temple of Aesculapius, the sick that they might be healed, and hung in them, as in the temples of the gods, sacred gifts of silver and gold. Their graves were, as Chrysostom says, more splendidly adorned and more frequently visited than the palaces of kings.
Banquets were held there in their honor, which recall the heathen sacrificial feasts for the welfare of the manes. Their relics were preserved with scrupulous care, and believed to possess miraculous virtue.
Earlier, it was the custom to pray for the martyrs (as if they were not yet perfect) and to thank God for their fellowship and their pious example. Now such intercessions for them were considered unbecoming, and their intercession was invoked for the living.
This invocation of the dead was accompanied with the presumption that they take the deepest interest in all the fortunes of the kingdom of God on earth, and express it in prayers and intercessions.
This was supposed to be warranted by some passages of Scripture, like Luke xv. 10, which speaks of the angels (not the saints) rejoicing over the conversion of a sinner, and Rev. viii. 3, 4, which represents an angel as laying the prayers of all the saints on the golden altar before the throne of God.
…the New Testament…furnishes not a single example of an actual invocation of dead men…But the New Testament expressly rebukes the worship of the angels (Col. ii. 18; Rev. xix. 10; xxii. 8, 9), and furnishes not a single example of an actual invocation of dead men; and it nowhere directs us to address our prayers to any creature. Mere inferences from certain premises, however plausible, are, in such weighty matters, not enough.
The intercession of the saints for us was drawn as a probable inference from the duty of all Christians to pray for others, and the invocation of the saints for their intercession was supported by the unquestioned right to apply to living saints for their prayers, of which even the apostles availed themselves in their epistles.
But here rises the insolvable question: How can departed saints hear at once the prayers of so many Christians on earth, unless they either partake of divine omnipresence or divine omniscience?
And is it not idolatrous to clothe creatures with attributes which belong exclusively to Godhead? Or, if the departed saints first learn from the omniscient God our prayers, and then bring them again before God with their powerful intercessions, to what purpose this circuitous way? Why not at once address God immediately, who alone is able, and who is always ready, to hear His children for the sake of Christ?
Augustine felt this difficulty, and concedes his inability to solve it. He leaves it undecided, whether the saints (as Jerome and others actually supposed) are present in so many places at once, or their knowledge comes through the omniscience of God, or finally it comes through the ministry of angels.
He already makes the distinction between latreiva, or adoration due to God alone, and the invocatio (douleiva) of the saints, and firmly repels the charge of idolatry, which the Manichaean Faustus brought against the catholic Christians when he said: “Ye have changed the idols into martyrs, whom ye worship with the like prayers, and ye appease the shades of the dead with wine and flesh.”
Augustine asserts that the church indeed celebrates the memory of the martyrs with religious solemnity, to be stirred up to imitate them, united with their merits, and supported by their prayers, but it offers sacrifice and dedicates altars to God alone.
Our martyrs, says he, are not gods; we build no temples to our martyrs, as to gods; but we consecrate to them only memorial places, as to departed men, whose spirits live with God; we build altars not to sacrifice to the martyrs, but to sacrifice with them to the one God, who is both ours and theirs.
But in spite of all these distinctions and cautions, which must be expected from a man like Augustine, and acknowledged to be a wholesome restraint against excesses, we cannot but see in the martyr-worship, as it was actually practiced, a new form of the hero-worship of the pagans.
Nor can we wonder in the least. For the great mass of the Christian people came, in fact, fresh from polytheism, without thorough conversion, and could not divest themselves of their old notions and customs at a stroke.