The Saints participate in the life of the Holy Spirit who is formless

And since many saints participate in the Holy Spirit, He cannot therefore be understood to be a body, which being divided into corporeal parts, is partaken of by each one of the saints; but He is manifestly a sanctifying power, in which all are said to have a share who have deserved to be sanctified by His grace.

Origen
On First Principles, Book I
Chapter I

Perpetua prays for her deceased brother

“After a few days, whilst we were all praying, on a sudden, in the middle of our prayer, there came to me a word, and I named Dinocrates; and I was amazed that that name had never come into my mind until then, and I was grieved as I remembered his misfortune. And I felt myself immediately to be worthy, and to be called on to ask on his behalf. And for him I began earnestly to make supplication, and to cry with groaning to the Lord.

…This Dinocrates had been my brother after the flesh, seven years of age who died miserably with disease—his face being so eaten out with cancer, that his death caused repugnance to all men.  For him I had made my prayer, and between him and me there was a large interval, so that neither of us could approach to the other.

…But I trusted that my prayer would bring help to his suffering; and I prayed for him every day until we passed over into the prison of the camp, for we were to fight in the camp-show. Then was the birth-day of Geta Cæsar, and I made my prayer for my brother day and night, groaning and weeping that he might be granted to me.

…Then I understood that he was translated from the place of punishment.

 

The Passion of the Holy Martyrs Perpetua and Felicitas
Chapter II

Communion with the Lord through the martyrs

And we therefore, what we have heard and handled, declare also to you, brethren and little children, that as well you who were concerned in these matters may be reminded of them again to the glory of the Lord, as that you who know them by report may have communion with the blessed martyrs, and through them with the Lord Jesus Christ, to whom be glory and honour, for ever and ever.

The Passion of the Holy Martyrs Perpetua and Felicitas 
Preface

The departed saints pray for us by being joined together with us by God

Now one of these excellences in the strictest sense according to the divine word is love for one’s neighbor, and this accordingly we are compelled to think of as possessed in a far higher degree by saints already at rest than by those who are in human weakness and wrestle on along with the weaker. It is not only here that “if one member suffers, all the members suffer with it and if one member is glorified, all the members rejoice with it” in the experience of those who love their brethren, for it beseems the love also of those who are beyond the present life to say “I have anxiety for all the churches:

Who is weak and I am not weak? Who is made to stumble and I do not burn?” Especially when Christ avows that according as such one of the saints may be weak, He is weak in like manner, and in prison and naked and a stranger and hungry and athirst. For who that reads the gospel is ignorant that Christ, in taking on himself whatever befalls believers, counts their sufferings His own?

And if angels of God came to Jesus and ministered to Him, and if we are not to think of the ministry of the angels to Jesus as having been limited to the brief space of His bodily sojourn among men while He was still in the midst of believers not as one that reclined at table but as one that ministered, how many angels, I wonder, must now be ministering to Jesus when He would “bring together the Children of Israel one by one” and gather them from the dispersion, saving those who fear God and call upon Him, and must be cooperating more than the apostles in the increase and enlargement of the church! Thus in John certain angels are spoken of in the Apocalypse as actually presiding over the churches.

Not in vain do angels of God ascend and descend unto the Son of Man, beheld of eyes that have been enlightened with the light of knowledge. In the very season of prayer, accordingly, being reminded by the suppliant of his needs, they satisfy them as they have ability by virtue of their general commission. To further the acceptance of our view we may make use of some such image as the following in support of this argument.

Suppose that a righteously minded physician is at the side of a sick man praying for health, with knowledge of the right mode of treatment for the disease about which the man is offering prayer. It is manifest that he will be moved to heal the suppliant, surmising, it may well be not idly, that God has had this very action in mind in answer to the prayer of the suppliant for release from the disease. Or suppose that a man of considerable means, who is generous, hears the prayer of a poor man offering intercession to God for his wants. It is plain that he, too, will fulfil the objects of the poor man’s prayer, becoming a minister of the fatherly counsel of Him who at the season of the prayer had brought together him who was to pray and him who was able to supply and by virtue of the rightness of his principles, incapable of overlooking one who has made that particular request.

As therefore we are not to believe that these events are fortuitous, when they take place because He who has numbered all the hairs of the head of saints, has aptly brought together at the season of the prayer the hearer who is to be minister of His benefaction to the suppliant and the man who has made his request in faith; so we may surmise that the presence of the angels who exercise oversight and ministry for God is sometimes brought into conjunction with a particular suppliant in order that they may join in breathing his petitions.

Nay more, beholding ever the face of the Father in heaven and looking on the Godhead of our Creator, the angel of each man, even of “little ones” within the church, both prays with us, and acts with us where possible, for the objects of our prayer.

Origen
On Prayer
Chapter VI

Origen quotes the Deuterocanon in support of the Intercession of Saints

But these pray along with those who genuinely pray—not only the high priest but also the angels who “rejoice in heaven over one repenting sinner more than over ninety-nine righteous that need not repentance,” and also the souls of the saints already at rest. Two instances make this plain. The first is where Raphael offers their service to God for Tobit and Sarah. After both had prayed, the scripture says, “The prayer of both was heard before the presence of the great Raphael and he was sent to heal them both,” and Raphael himself, when explaining his angelic commission at God’s command to help them, says:

“Even now when you prayed, and Sarah your daughter-in-law, I brought the memorial of your prayer before the Holy One,” and shortly after, “I am Raphael, one of the Seven angels who present the prayers of saints and enter in before the glory of the Holy One. Thus, according to Raphael’s account at least, prayer with fasting and almsgiving and righteousness is a good thing.

The second instance is in the Books of the Maccabees where Jeremiah appears in exceeding “white haired glory” so that a wondrous and most majestic authority was about him, and stretches forth his right hand and delivers to Judas a golden sword, and there witnesses to him another saint already at rest saying, “This is he who prays much for the people and the sacred city, God’s prophet Jeremiah.” For it is absurd when knowledge, though manifested to the worthy through a mirror and in a riddle for the present, is then revealed face to face not to think that the like is true of all other excellences as well, that they who prepare in this life beforehand are made strictly perfect then.

Origen
On Prayer
Chapter VI

The departed saints join in our prayers

Yet there is a certain helpful charm in a place of prayer being the spot in which believers meet together. Also it may well be that the assemblies of believers also are attended by angelic powers, by the powers of our Lord and Savior himself, and indeed by the spirits of saints, including those already fallen asleep, certainly of those still in life, though just how is not easy to say.

And if Paul, while still wearing the body, believed that he assisted in Corinth with his spirit, we need not abandon the belief that the blessed departed in spirit also, perhaps more than one who is in the body, make their way likewise into the churches. For that reason we ought not to despise prayer in churches, recognizing that it possesses a special virtue for him who genuinely joins in.

Origen
On Prayer
Chapter XX

The souls of the departed live on in an intermediate state before heaven

It must therefore be evident to every man of intelligence who has ever heard of the Elysian fields, that there is some determinate place called Abraham’s bosom, and that it is designed for the reception of the souls of Abraham’s children, even from among the Gentiles (since he is “the father of many nations,” which must be classed amongst his family), and of the same faith as that wherewithal he himself believed God, without the yoke of the law and the sign of circumcision. This region, therefore, I call Abraham’s bosom. Although it is not in heaven, it is yet higher than hell, and is appointed to afford an interval of rest to the souls of the righteous, until the consummation of all things shall complete the resurrection of all men with the “full recompense of their reward.”

Tertullian
The Five Books Against Marcion, Book IV
Chapter XXXIV

The thoughts of the saints cover the world

And should any one say that the voice does not reach God, but is rolled downwards in the air, yet the thoughts of the saints cleave not the air only, but the whole world. And the divine power, with the speed of light, sees through the whole soul.

Clement of Alexandria
The Stromata, Book VII
Chapter VII