This is a case where America Unearthed made a claim that was not exactly wrong but not exactly right, and did nothing to acknowledge the great uncertainty surrounding the material.
This identification lets advocates adopt blood rites attributed to the cult of Attis for the god Mithras. But how did that happen? I think you’ll find this passage from Sir James Frazer’s monumental but flawed masterwork, The Golden Bough, summarizing the Christian writer Prudentius, suspiciously similar to what Joe Rose said:
In the baptism the devotee, crowned with gold and wreathed with fillets, descended into a pit, the mouth of which was covered with a wooden grating. A bull, adorned with garlands of flowers, its forehead glittering with gold leaf, was then driven on to the grating and there stabbed to death with a consecrated spear. Its hot reeking blood poured in torrents through the apertures, and was received with devout eagerness by the worshiper on every part of his person and garments, till he emerged from the pit, drenched, dripping, and scarlet from head to foot, to receive the homage, nay the adoration, of his fellows—as one who had been born again to eternal life and had washed away his sins in the blood of the bull. (Frazer, Golden Bough, third ed., part 4, vol. 2, Adonis, Attis, Osiris, 1919, p. 274)
With these words Aristo tried to clear himself, but they moved the godless persecutor of the Christians not at all, and he rushed more and more into a mad rage. He asked whether that was someone else's blood which bespattered Romanus, or whether it flowed from a wound of his own. To this Romanus answered: "Here I am before you. This is truly my own blood, not that of an ox. Do you realise, unhappy pagan, the blood I speak of, — the sacred blood of your ox, in the sacrificial slaughter of which you soak yourselves?" The high priest, you know, goes down into a trench dug deep in the ground to be made holy, wearing a strange headband, his temples bound with its fillets for the solemnity. Above him they lay planks to make a stage, leaving the timber-structure open, with spaces between; and then they cut and bore through the floor, perforating the wood in many places with a sharp-pointed tool so that it has a great number of little openings. Hither is led a great bull with a grim, shaggy brow, wreathed with garlands of flowers about his shoulders and encircling his horns, while the victim's brow glitters with gold, the sheen of the plates tinging his rough hair. When the beast for sacrifice has been stationed here, they cut his breast open with a consecrated hunting-spear and the great wound disgorges a stream of hot blood, pouring on the plank-bridge below a steaming river which spreads billowing out. Then through the many ways afforded by the thousand chinks it passes in a shower, dripping a foul rain, and the priest in the pit below catches it, holding his filthy head to meet every drop and getting his robe and his whole body covered with corruption. Laying his head back he even puts his cheeks in the way, placing his ears under it, exposing lips and nostrils, bathing his very eyes in the stream, not even keeping his mouth from it but wetting his tongue, until the whole of him drinks in the dark gore. After the blood is all spent and the officiating priests have drawn the stiff carcase away from the planking, the pontiff comes forth from his place, a grisly sight, and displays his wet head, his matted beard, his dank fillets and soaking garments. Defiled as he is with such pollution, all unclean with the foul blood of the victim just slain, they all stand apart and give him salutation and do him reverence because the paltry blood of a dead ox has washed him while he was ensconced in a loathsome hole in the ground.
Nevertheless, on this sole authority, scholars from the eighteenth century until the 1980s assumed that a primeval rite of baptism by blood bath was performed for the worshipers of Attis, despite no archaeological evidence for it in the form recorded by Prudentius. The oft-quoted claim—repeated by Frazer in the Golden Bough, but predating him by several decades—that Firmicus Maternus (De errore profanarum religionem 28.8 [27.8 in some editions]) supported the bull blood rite is false; he merely claims that pagans are morally stained by their blood rites of animal sacrifice (and the blood that falls on them while killing animals), not that they physically bathe in the blood. The only other source for the rite, a poem called Contra paganos, is believed to contain material derivative of Prudentius and therefore is no independent confirmation. Rose is unaware of modern interpretations, but at any rate, this rite, even if believed at face value, describes the rites of Attis and Cybele, not Mithras. As I pointed out before, Tertullian’s De baptismo (5.1) makes plain that the initiates of Mithras were baptized with water.
Frazer’s Golden Bough is also responsible for popularizing the incorrect claim that the Mithras cult was similar to that of Cybele and to early Christianity; this meant that Frazer could imagine blood baptisms for Mithras. This claim goes back to the eighteenth century and derives from the scholarly assumption that the slaying of the cosmic bull by Mithras had the same origin as the bull-slaying rites of Cybele, and also from some sculptures from Asia Minor that seem to identify Attis with Mithras (both gods are young men who wear Phrygian caps, for example). The same claims allows Rose to associate the blood rites with Mithras. It was actually a man named Arthur Darby Nock who made Frazer’s implication of Mithraic blood baptisms explicit, arguing that the Mithras cult was Phrygian rather than Persian or Roman and therefore conducted Attis blood rites; he was refuted systematically several times, as Alvar explained. As we have seen, the Mithras cult baptized with water, and no archaeological evidence of any pits for showering in bull’s blood exists. But, even taking everything at face value, the blood baptism was a rite of Attis rather than Mithras and therefore of no relevance to any Mithras artifacts or inscriptions found in Oklahoma.