I’d like to take a look at the views of Origen (c. 184 – c. 253) on the invocation and intercession of saints, and determine whether it is consistent with the Catholic view that such things were widely taught by the Church Fathers. He wrote:

For we indeed acknowledge that angels are ministering spirits, . . . and that they ascend, bearing the supplications of men, to the purest of the heavenly places in the universe, or even to supercelestial regions purer still; and that they come down from these, conveying to each one, according to his deserts, something enjoined by God to be conferred by them upon those who are to be the recipients of his benefits. . . . For every prayer, and supplication, and intercession, and thanksgiving, is to be sent up to the Supreme God through the High Priest, who is above all the angels, the living Word and God. And to the Word Himself shall we also pray and make intercessions, and offer thanksgivings and supplications to Him, if we have the capacity of distinguishing between the proper use and abuse of prayer. . . . 

And it is enough to secure that the holy angels of God be propitious to us, and that they do all things on our behalf, that our disposition of mind towards God should imitate as far as it is within the power of human nature the example of these holy angels, who again follow the example of their God; ... (Contra Celsum, V, 4-5; my bolding and italics)

Two things are going on here, that are complementary, not contradictory; “both/and” and not “either/or”:

1) Prayers ultimately go to God who decides how he wants to address them: by granting or refusing the request.

2) Angels, saints, and fellow human beings (whether alive on the earth or inhabiting the next world) may intercede and present our requests, prayers, supplications, petitions to God on our behalf.

Origen (unlike many if not most Protestants) sees no contradiction or disharmony or discord at all between nos. 1 and 2, and so he expresses both, casually assuming that both things are simultaneously true. 

The bolded parts of the above citation show that Origen believes that angels intercede for us to God. The italicized parts show that God is the ultimate recipient and granter or denier of the wishes expressed in prayers. 

The key phrase relating to angelic intercession here is “bearing the supplications of men.” Supplications are prayers. The English word appears six times in the RSV in the New Testament, and its meaning is apparent:

Ephesians 6:18 (RSV) Pray at all times in the Spirit, with all prayer and supplication. To that end keep alert with all perseverance, making supplication for all the saints, (cf. Philippians 4:6; 1 Timothy 2:1, 5:5; Hebrews 5:7)

Angel are not only involved in the prayer to God but also in its fulfillment from God. They’re doing precisely what we see in Revelation 8:4 (“and the smoke of the incense rose with the prayers of the saints from the hand of the angel before God.”).

These angels and dead saints (as in Revelation 5) are somehow involved in the process of these prayers “getting to God.” That’s precisely the intercession of saints and angels. Why is that? If prayer is supposed to be solely between a Christian believer and his or her God, with no intermediary, why do “the prayers of the saints” rise to God “from the hand of the angel”? 

Likewise, what are the “elders” (usually taken to be dead men) doing with “the prayers of the saints” (Revelation 5:8)? Why are they involved at all? They are because God wills it.

But we don’t pray to angels and saints as if they were the ones who granted or denied our prayers, and had the power to do so (the ultimate recipients). The very terms Catholics use presuppose this: intercession of the saints (not, “saints granting prayers”): just as we intercede for each other on earth. 

In any event, it is praying to God when we ask a holy person or an angel to pray for us. We ask them to pray, and they intercede to God on our behalf.  

It’s sort of like asking a friend who is in the right spot to “intercede” and speak on our behalf with regard to obtaining some job. Then if we get it we may thank them by saying, “you got me the job!” In a sense they did, but of course, ultimately, it was the employer who decided.

Catholics invoke saints and ask them to intercede for us because they are more alive than we are and certainly more spiritually powerful, and because James 5:16 states that “the prayer of a righteous man has great power in its effects.” 

Origen, in his most well-known work, Contra Celsum, explicitly endorses the notion of saints and angels praying for and with us (strongly implying also our invocation of them), in Book VIII, 64:

[W]hen we have the favor of God, we have also the good-will of all angels and spirits who are friends of God. For they know who are worthy of the divine approval, and they are not only well disposed to them, but they co-operate with them in their endeavors to please God: they seek his favor on their behalf; with their prayers they join their own prayers and intercessions for them. 

We may indeed boldly say, that men who aspire after better things have, when they pray to God, tens of thousands of sacred powers upon their side. These, even when not asked, pray with them, they bring succor to our mortal race,. . . (my bolding)

Note, especially, how Origen casually states: “These [departed saints and angels], even when not asked, pray with them . . .” The use of “even” shows that Origen assumes that it is also the case that they pray for us when asked; i.e., invoked.