The Potential Significance of Anarthrous Theon in John 1:18

Greek (SBLGNT): θεὸν οὐδεὶς ἑώρακεν πώποτε· μονογενὴς θεὸς ὁ ὢν εἰς τὸν κόλπον τοῦ πατρὸς ἐκεῖνος ἐξηγήσατο.

There is plenty that one could say about the Greek of John 1:18, but notice that the accusative form θεὸν appears as the initial word, and the verb ἑώρακεν modifies the noun while οὐδεὶς and πώποτε emphasize that no one has seen God "at any time" (at all). 

However, just what is significant about the anarthrous and initial word, θεὸν? Why did John produce this construction?

John F. McHugh (John 1:1-4):

The absence of the article before θεόν implies that no one had ever (previously) seen God qua God,55 though they might have ‘seen’ him under shadows and figures at Mamre, at the burning bush, or in a vision (Gen 18; Exod 3; Isa 6). That is, no one had ever seen and known God in the way one knows oneself or another human being (cf. Exod 33.18-20). 

Merrill C. Tenney (John: The Expositor's Bible Commentary, page 34):

The noun God (theon) has no article in the Greek text, which indicates that the author is presenting God in his nature of being rather than as a person. "Deity" might be a more accurate rendering. The meaning is that no human has ever seen the essence of deity. God is invisible, not because he is unreal, but because physical eyes are incapable of detecting him. The infrared and ultraviolet rays of the light spectrum are invisible because the human eye is not sensitive enough to register them. However, photographic plates or a spectroscope can make them visible to us. Deity as a being is consequently known only through spiritual means that are able to receive its (his) communications.

Whitfield Mowbray, D.D. (Durham University Thesis for M.Litt., page 83):

It is pre-eminently as FATHER that the Word reveals God (anarthrous, in v.18, suggesting Nature rather than Person: 'God as God').