Addition after Comment : The title header does not provide sufficient space to word the question without ambiguity. Thus :
If the one born in Bethlehem was ‘a creature’ (as says Arius) and yet did not sin, then why was the first humanity (Adam) not so created (according to the words and doctrine of Arius ?) ...
... is the full and unambiguous title.
In an open letter to the Alexandrians, the signatories to the Council of Nicaea state the following of the doctrine of Arius :
In the first place, then, the impiety and guilt of Arius and his adherents were examined in the presence of our most religious emperor Constantine, and it was unanimously decided that his impious opinion should be anathematized, with all the blasphemous expressions he has uttered, in affirming that "the Son of God sprang from nothing" and that "there was a time when he was not." He said moreover that "the Son of God, because possessed of free will, was capable either of vice or virtue," and he called him a creature and a work. All these sentiments the holy synod has anathematized, having scarcely patience to endure the hearing of such an impious opinion—or rather madness—and such blasphemous words.
(Ecclesiastical History of Socrates Scholasticus I:9.)
[As has been noted, and as is well known, it is difficult to source documents from Arius himself, but easier to cite from his opponents, who quote him extensively.]
Elsewhere, Trinitarian doctrine is quite clear in stating that He who is called ‘word’ was ‘with God’ ‘in the beginning ‘ (John 1:1), and ‘God was the word’ (John 1:1, literal) ; that ‘the life the eternal which was with the Father’ (1 John 1:2, literal) is ‘manifested’ (1 John 1:2) which ‘manifestation‘ is also named ‘the Son of God‘ (1 John 3:8); and that ‘God was manifest in the flesh’ (1 Timothy 3:16, TR/KJV).
Trinitarian doctrine clearly states that Jesus of Nazareth is not merely a human person, whose existence began in Bethlehem, on earth, in flesh, but that this humanity is a ‘manifestation’ of Him who is divine, eternal and co-equal in deity with the Father.
Trinitarian doctrine makes it clear that the first man was of earth, earthy, only ; mere flesh and blood, with breath breathed into him, no more. But Jesus of Nazareth is a new creation, humanity manifesting God himself ; humanity and deity in one unique Person.
Thus Trinitarian doctrine is clear that Jesus is not another created Adam, a second creation of mankind.
But Arius did so state. Thus Arius seems to me to be finding fault with the Creator. For if Adam is a man of sin and death (as scripture unarguably conveys, and as every penitent soul admits) then the one born in a manger in Bethlehem was a ‘second attempt’ ; surely.
[Note after comment : I am speaking of the manifested humanity, not of any 'prior' (presumably angel-like) spirit sort of existence. The question is about manifested humanity and its propensities within the created sphere.]
For if it be the case that the babe in the manger is ‘capable of virtue or vice’ and is a ‘created creature’ and yet did not do any vice, but only ever did good ; then why did the Creator not make this creature to begin with ? Why the failure of Adam ?
For Trinitarian doctrine clearly states that the liability inherent in any creation (and therefore in the best of all possible creations) is, firstly, the creature itself (that it is just that - creature) and, secondly, the inevitable presence in the creation of a certain knowledge (the knowledge of good and evil) which knowledge is at once both so irresistibly tempting to the creature yet so utterly and fatally catastrophic to the creature.
But does Arius’ doctrine convey that truth ? For if the one come of woman in Bethlehem is merely another creature, and that creation was such that he did not fail or fall, yet he could do either, it is said, then why - O why ? - did not the one whom Arius calls ‘Creator’ create that perfection in the first place ?
What do we know of the doctrine of Arius that would demonstrate he had a solution to this doctrinal problem ?
Scripture quotations are from the KJV (1769) or from the literal translation provided by the Englishman's Greek New Testament (1877).