This is a harder question than it might at first seem.
Accounts and physical descriptions of angels in both canonical and non-canonical sources vary wildly. Sometimes, they are indistinguishable from men, while other times, they are animal-faced creatures with six wings and many eyes. Therefore, the question of a true or accurate angelic form was a contentious subject among early church fathers. Thinkers such as St. Augustine (354-430 CE), Pseudo-Dionysius (c. 5th-6th century CE), and John of Damascus (675-749 CE) popularized the idea that angels are not material creatures in the way that humans are, and that their incorporeal essence–often likened to fire in its quality–render humans incapable of perceiving them as they really are. Because of this, they reasoned, angels are able to take on a metaphorical, substantial form in order for humans to be able to comprehend aspects of angelic nature that they otherwise would not have access to.
They are circumscribed: for when they are in the Heaven they are not on the earth: and when they are sent by God down to the earth they do not remain in the Heaven. They are not hemmed in by walls and doors, and bars and seals, for they are quite unlimited. Unlimited, I repeat, for it is not as they really are that they reveal themselves to the worthy men to whom God wishes them to appear, but in a changed form which the beholders are capable of seeing. For that alone is naturally and strictly unlimited which is un-created. For every created thing is limited by God Who created it... Seeing that they are minds they are in mental places, and are not circumscribed after the fashion of a body. For they have not a bodily form by nature, nor are they extended in three dimensions.
John of Damascus, 19b
For the mind can by no means be directed to the spiritual presentation and contemplation of the Celestial Hierarchies unless it use the material guidance suited to it, accounting those beauties which are seen to be images of the hidden beauty, the sweet incense a symbol of spiritual dispensations, and the earthly lights a figure of the immaterial enlightenment.
Pseudo-Dionysius, 149
This rectified the issues that arose from such inconsistent biblical accounts of angelic form, but it posed another question: how and why did the angels throughout scripture choose the specific forms they took? To understand this, it is important to understand what angels fundamentally are. The biblical role of an angel is to act as a divine messenger of God. Angels are the intermediaries between humans and the heavenly realm, and their purpose is to communicate the will of God to the people deemed worthy of receiving this message. From the Old Testament angels bringing visions to the prophets to the Archangel Gabriel informing Mary of her Pregnancy in the Gospel According to Luke, Christian Doctrine is brimming with angels bearing divine messages to God's chosen. The theologian whose works we attribute to Dionysius the Areopagite, or Pseudo-Dionysius, wrote a treatise on angelic hierarchy that became hugely influential in the field of angelology. In this treatise, "On the Celestial Hierarchy," he discusses angelic appearance as an extension of their Biblical role. Just as through their actions, angels work to bring humans toward divine enlightenment, Pseudo-Dionysius proposed that the form angels take are intentionally chosen to communicate divine insight to their viewer. Angelic appearance does not correlate to any concept of accurate angelic form, which is inexpressible and unknowable; instead, it is a visual metaphor for their nature and their proximity to God, which elevates those who are worthy enough to see and understand.
Theology, in its sacred utterances concerning the formless Intelligences, does indeed use poetic symbolism, having regard to our intelligence, as has been said, and providing a means of ascent fitting and natural to it by framing the sacred Scriptures in a manner designed for our upliftment.
Pseudo-Dionysius, 152
Images are comprehensible to inferior human perception. Human perception is able to approach the image at a spiritual and not a literal level, in order to unlock the meaning behind its enigmatic demonstration of angelic nature.
Peers, 114
Therefore, while angels are not material, they may present themselves as material beings which metaphorically represent an aspect of their incomprehensible true form to bring us a step closer to enlightenment about the true nature of angelic form, and therefore, one step closer to knowledge of God.
Peers, 117
Early depictions of angelic form in Christian art drew on numerous cultural and religious trends, from the conflicting accounts in scripture to earlier pagan artwork, and in the roughly 2000 years that have passed since the dawn of Christianity, the idea of "accuracy" in regards to angelic form has shifted, but is no more of a realistic question than it was at the time that Pseudo-Dionysius was writing. At best, authenticity in religious art is subjective, and at worst, it is reductive and unhelpful. For this reason, when discussing depictions of angels in Christian art, I find it useful to apply the work of Pseudo-Dionysius. You should not approach any given image with the question, "Is this representation accurate?" (Accurate to what? To Biblical angels? To earlier Christian depictions of angels?) A better question to ask instead is, "In what way is this representation an expression of angelic nature, angelic divinity, and angelic proximity to God?"
- John of Damascus. (n.d.). John of Damascus: Exposition of the Orthodox Faith. In P. Schaff (Ed.), Nicene and Post-Nicene. Fathers (Series 2 ed., Vol. 9).
https://ccel.org/ccel/schaff/npnf209/npnf209.
- Peers, G. (2001). Subtle bodies: Representing Angels in Byzantium. Berkeley and Los Angeles, California. University of California Press.
- Pseudo-Dionysius. (n.d.). Celestial Hierarchy. https://ccel.org/ccel/dionysius/celestial/celestial.
https://ccel.org/ccel/schaff/npnf209/npnf209.
- Peers, G. (2001). Subtle bodies: Representing Angels in Byzantium. Berkeley and Los Angeles, California. University of California Press.
- Pseudo-Dionysius. (n.d.). Celestial Hierarchy. https://ccel.org/ccel/dionysius/celestial/celestial.