Is William Barclay a respected theologian in mainstream Christianity? William Barclay
A prolific author of 70 books, including commentaries on each New Testament books, is a well known and often quoted scholar.
However some have written articles about him and challenge his Christianity based on the facts that he did not accept the miracles of Jesus, did not believe in the preexistence of Jesus, was a universalist and as can be seen in the quote below did not hold to classic views on the trinity.

Nowhere does the New Testament identify Jesus as God. Jesus did not say, &quot;He who has seen me has seen God.&quot; He said, &quot;He who has seen me has seen the Father.&quot; There are attributes of God I do not see in Jesus. I do not see God's omnipotence in Jesus, for there are things which Jesus did not know. I do not see God's omnipotence in Jesus for there are things which Jesus could not do (William Barclay, A Spiritual Biography, 1977 Edition, p. 56).

Is William Barclay an esteemed scholar recognized by main stream Christianity, or are his views considered heretical in many ways?
Professor William Barclay has been somewhat misrepresented by some who object to the fact that he held to some ‘liberal’ ideas (theologically speaking). This Greek scholar and Classics expert certainly was at odds with many Christians on quite a few points, but not with regard to Jesus being God incarnate, the Second Person of the Trinity. Whatever critics say about this man (who is not around to defend himself), he cannot be charged with lending any support to anti-trinitarian religionists who deny that “the Word was God”. I am not interested in defending the theology of Barclay, nor am I versed in his writings, but I do object to those who try to use partial quotes from his writings regarding the deity of Christ, to either claim he wasn’t “a real Christian” at all, or that he actually agrees with anti-trinitarians who deny that “the Word was God”. He never did support that school of thought.  It is scholastically dishonest to cite him in such a way that he appears to claim that Jesus was not God incarnate.
The article quoted from in the OPs answer to his own question is interesting in that the writer (Dudley Ross) criticises Barclay for being “a liberal”; for being less than enamoured of evangelicals in the 20th century who were viewed as being conservatives, whereas he viewed himself as a liberal evangelical; for admitting to having ‘borrowed ideas’ and not original ones of his own. Ross quotes Barclay as making the allegation, &quot;The phrase `born of woman' has nothing to do with the Virgin Birth.&quot; I know of some trinitarians who would agree, because that phrase is never said of Jesus in the Bible – it is said of John the Baptist.
Ross does more partial quoting to accuse Barclay of this theological error: “Barclay really did not believe that Jesus was God.” The partial quote Ross uses does not favour us with the context or if Barclay then went on to add how Jesus was, indeed, God, despite those truthful points from scripture he had detailed. Like Barclay, trinitarians know that they cannot quote a verse that says “Jesus is God”, as if Jesus was the entire summation of God when the Bible is clear that the Father is not the Son, and the Son is not the Father, yet both are equally God in their divine Being. So, when Barclay highlights how Jesus was sent by the Father, and prayed to Father God, and was devoted to doing the will of his Father, that would be something trinitarians agree with.
When it comes to ‘proof’ that Barclay did not believe Jesus was God, the best Ross can come up with is to refer to another person, a liberal -

“Thayer was a Unitarian, and the errors of this sect occasionally come
through in the expalatory [sic?] notes. The reader should be alert for
both subtle and blatant denials of such doctrines as the Trinity
(Thayer regarded Christ as a mere man and the Holy Spirit as an
impersonal force emanating from God)…”

But Barclay has never stated that Christ is “a mere man” nor that the Holy Spirit is “an impersonal force emanating from God.” If Barclay had said any such things, you may be sure that direct quotes would have been given. Instead, we have an attempt to tar Barclay with the same Unitarian brush that smears Thayer.
Now let Barclay speak up in protest at those who deny that Jesus is God, and who claim that the Holy Spirit is simply the power of God. Before he died, he took a group to task for trying to partially quote him to imply that he supported them. He wrote,

“The deliberate distortion of truth by this sect is seen in their New
Testament translations.  John 1:1 is translated: ‘…the Word was a
god’, a translation which is grammatically impossible… It is
abundantly clear that a sect which can translate the New Testament
like that is intellectually dishonest.”

And that is the issue here – intellectual dishonesty. It is not about the extent of Barclay’s theological liberalism on matters other than the deity of Christ. I do not care how liberal Barclay might have been because I am not a follower of Barclay nor was I taught by him. I am only concerned with the issue of whether anti-trinitarians can cite him honestly as one of their own, or whether Barclay held to a higher, divine view of the person of Christ than they have. Many anti-trinitarians quote from Unitarians in support of their views, but the question is – can they honestly quote Barclay as a supporter of their anti-trinitarian views? I think not.
I have no intention of going on to defend Barclay by quoting copiously from his statements about the deity of Christ. It should suffice that he objected to one anti-trinitarian group trying to claim his views in support of their own on that matter. Nor will it do to claim that the question as to whether he is “a respected theologian in mainstream Christianity” links in with the claim that for many on this site “anyone who denies the trinity is not a Christian since a Christian must accept the Creeds”. Many on this site would point out that acceptance of the trinity equates with accepting what the Bible says on the matter. The biblical doctrines of God, Christ, and the Holy Spirit can only ever come from the Bible. The Bible came first, creeds came later. The first century Christians wrote of how they viewed Christ, and all who call themselves Christians must adhere to what their scriptural writings declare, for therein is the revelation of God, by the Holy Spirit, as to just who this Jesus really is.
Some people in mainstream Christianity (whatever that is) will view Barclay as being a respected theologian; others will disagree. At the end of the day, people are entitled to disrespect Barclay for holding to views on many points that they disagree with, but they should not go on to approve of what he says on the person of Christ, thinking he’s anti-trinitarian when he was no such thing. Those who quote Barclay out of context on that doctrine run the risk of being scholastically dishonest.