A major reason for me doubting Paul is his use of the terms "likeness," "form," and "appearance" as it relates to Jesus' human nature. It's become my view that Paul's language is categorically antichrist. There are two primary verses we'll consider and then others we must bring into our study in order to understand the use of the term. First, let's define who and what antichrist is.
The importance of this can't be understated. The Apostle John says the following that we must consider:
And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
John says that Jesus came in the flesh; he didn't say that it looked like he did, or that Jesus came into the flesh, or that Jesus no longer possess it because he's ascended and glorified. The problem with Trinitarianism and others who believe in a pre-existent Jesus is that the language suggests that's what happened. Trinitarians, Modalists, and Arians think "Jesus" somehow pre-existed as someone else before his "incarnation," wrongfully identifying the "Word" as a person. These same groups typically identify him as the Angel of the Lord, a sort of "spirit being." Arians like Jehovah's Witnesses and some non-Trinitarian SDAs are more honest with that claim and apply the name "Michael" to him and rank him as an Archangel.
Either way, this "pre-existent Son of God" was at one point "without flesh" but then "came into" or "added" flesh in the "incarnation." The poetic way they say this is that "He added humanity." The problem is obvious: it's a pre-existent person who merely appeared in something they otherwise never would've had. He didn't originally have a "human nature," but it's something he was able to put on later. This isn't true to the definition of begetting, conceiving, or the idea of a man "come in the flesh" the way we're supposed to understand it in Matthew, Mark, Luke, and John. In short, Modalists, Trinitarians, and Arians--all who believe the son had a spiritual pre-existence--technically don't have a man being conceived in a womb; rather, they have a pre-existent spirit being donning a skin suit in order to appear human and experience some of our limitations. It's antichrist. Hence, my conclusion that Paul's teachings of Jesus are proto-Docetist and helped these heresies eventuate.
Many friends of mine are insistent that Paul never would've taught such a thing. After all, Paul "emphatically" asserts that Jesus was "a man" and had a "real" human nature. Let's consider an important aspect to Docetism that might shed light on Paul's language that we'll soon begin to examine. Docetism is "affirming that Christ did not have a real or natural body during his life on earth but only an apparent or phantom one" (Emphasis added). As we'll see, Paul's language implies exactly that! This is what I believe Paul taught. There are two verses that suggest this is, coupled with the fact that Paul never knew a physical Jesus--he only encountered an ascended and spiritualized one.
Rom 8:3 For what the law could not do in that it was weak through the flesh, God did by sending His own Son in the likeness of sinful flesh...
Phl 2:5 Let this mind be in you which was also in Christ Jesus,who, being in the form of God, did not consider it robbery to be equal with God, but made Himself of no reputation, taking the form of a bondservant, and coming in the likeness of men. And being found in appearance as a man, He humbled Himself and became obedient to the point of death, even the death of the cross.
I've underlined the relevant words which are the following in Greek:
"Likeness" in Greek is ὁμοίωμα, homoiōma (hom-oy'-o-mah), "From G3666; a form; abstractly resemblance: - made like to, likeness, shape, similitude."
Here are some other verses where the word is found:
Rom 1:23 and changed the glory of the incorruptible God into an image made like corruptible man and birds and four-footed animals and creeping things.
Rom 5:14 Nevertheless death reigned from Adam to Moses, even over those who had not sinned according to the likeness of the transgression of Adam, who is a type of Him who was to come.
Rom 6:5 For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection
Rev 9:7 The shape of the locusts was like horses prepared for battle. On their heads were crowns of something like gold, and their faces were like the faces of men.
We didn't sin as Adam did, we weren't crucified or raised from the dead as Jesus was, and the faces of the locusts only resemble human faces. So in each case, we have something that merely resembles something else and isn't the real thing. This is the word Paul chooses to describe the humanity of Jesus--it isn't a real human nature, but it appears to be.
Another important word Paul uses is "form," which in Greek is μορφή, morphē (mor-fay'), "Perhaps from the base of G3313 (through the idea of adjustment of parts); shape; figuratively nature: - form." It could be said that "form" and "likeness" are synonyms. They are communicating essentially the same idea: Jesus merely appeared in the nature, shape, or resemblance of a man.
The lengths people go to in their efforts to defend their champion apostle is startling, despite the obvious implication of this word. Let's quickly consider the Trinitarian and Socinian interpretations of Philippians 2:5 (which also would be used to understand Paul's statement in Romans 8:3).
Interpretations of Philippians 2:5
Trinitarians would interpret the verse as such: "Jesus was both God and man; he's in the form of God, and he's in the form of man--his two natures. He didn't think it was robbery to be equal with God because he was God. But when he came to earth, he simply added humanity and took on the form of a servant." So to the Trinitarian, Jesus has a Deity form ("divine nature") and a Human form, and this is considered to be the "greatest act of humility." Nevermind the complications that come with the "two-nature" theory, such as the idea that when Jesus dies, the Divine Nature (spirit) has to depart (because it can't die) and leaves the Human Nature on the cross. Keep in mind that to a Trinitarian, the Divine Nature is referred to as "God the SON," and the Human Nature is inevitably identified as "Jesus, the MAN." This is Docetism!
Biblical Unitarians, such as myself (when I defend such verses), would interpret the verse as such: "Just because Jesus is in the 'form' of God doesn't prove he's God. What that means is that he has divine prerogatives because he's God's uniquely begotten son. But, instead, he's definitely in the form of a servant, meaning that he didn't abuse his prerogatives. For example: he could've called legions of angels to destroy his enemies, but he didn't. Rather, he humbled himself and he went to trial and ultimately death. Everywhere, we see him serving rather than ruling while on earth." For the Unitarian, Jesus is on one hand "in the form of God" but "only in the sense that he's God's unique son" who has a potential to do more than he chose to because he's a servant, and this is the "greatest act of humility" and also provides a real example to follow.
Reflections of the interpretations
The Socinian, although capable of a decent interpretation, is committing an error. How is it that being in the form of God doesn't make him God, but being in the form of a servant makes him a man? They want one but not the other. If we're honest, we'll see Paul's parallelism is technically more logical for a Trinitarian's or Modalists' conclusion. Therefore, it's my conclusion that to be a consistent Unitarian, you must abandon Paul's conception. We shouldn't flirt with Docetism.
The problem in every sense is simply ignoring the definitions of these terms. Regardless of one's opinion on whether or not Jesus was actually God, the real issue is that Jesus is, according to Paul, only in the appearance and likeness and form of a man.
As mentioned at the outset, Paul may not have been a formal Docetist, but he certainly was a proto-Docetist, and the language he used in places throughout his letters is proof. As a result, Trinitarians, Modalists, and Arians--who're very keen on citing Paul--actually have, at the very least, a quasi-Docetist Christology, which is still antichrist. Paul's confusing understanding of Jesus' nature isn't surprising, since everywhere throughout his epistles he's obsessed with the belief that matter--especially the flesh--is evil and only the spiritual is good. More-so than any other New Testament person, Paul places a great deal of emphasis on the dual-nature idea of spirit verses flesh and listening to one's conscience for guidance, which are Gnostic ideas. He carries these themes throughout his arguments in his effort to establish himself as a pre-eminent apostle. But he isn't pre-eminent. It's time Biblical Unitarians re-consider the language used by Paul in his epistles and try the spirits to see whether they be of God.
 1 John 4:3, KJV
 Docetism, (from Greek dokein, “to seem”), Christian heresy and one of the earliest Christian sectarian doctrines, affirming that Christ did not have a real or natural body during his life on earth but only an apparent or phantom one. (Britannica)
 These verses are in the NKJV
This is an on going study and is subject to revisions.