Why were Pseudo-Dionysius's works accepted as authentic for so long? Pseudo-Dionysius was a 5th/6th century Christian theologian who claimed to be the Dionysius that Paul converted in Acts 17:34.  This claim was widely accepted for almost a millennium, which afforded his work significant influence in medieval Christianity.

I know that historiography has come a long way in the last 1500 years, but the idea that these works could have originated in the first or second century seems completely preposterous.  Why, for example, were none of the writings of "Dionysius" quoted in the prolific writings of Clement of Alexandria, Origen, Basil the Great, Gregory of Nyssa, or Gregory of Nazianzus?  Not to mention their complete absence in the Councils and in Western writers like Augustine.

I see from the Wikipedia article that "Dionysius" had some early skeptics, but a number of medieval figures argued for the early dating, such as Severus, John of Scythopolis, and Theodore of Raithu.  My question is: what arguments did these and other early-date proponents make that convinced medieval Christianity to consider these works authentic?  

I know that some of these writers had a vested interest in the authenticity of the works, but they must have made other arguments besides "I agree with Dionysius so he must be who he claims to be."  What evidence did they cite?
I think that a distinction needs to be drawn between authentic in the sense that the works were historically authentic (i.e. indeed written by the Dionysius of Acts) and authentic in the sense that they correctly reflect Apostolic teaching.

Eastern Christian theology is not based so much on the former as the latter.  The Pseudo-Dionysian corpus was not so much accepted because the latter Fathers thought it was written by Dionysius (which many seem to have believed), but rather because of the clarity with which it expressed certain truths which they held.  

To perhaps further shock, Pseudo-Dionysius continues to remain an important part of the written tradition of the Eastern Orthodox Church to this day, despite full awareness of the issues with its historical authenticity.  The entry on Dionysius in the text of one prominent Russian Orthodox theologian reads, for example:


  St. DIONYSIUS the Areopagite ---          A.D. 96
  
  A disciple of the Apostle Paul (Acts 17: 34), first Bishop of Athens,
  martyred in Paris; commemorated October 3. To him have been attributed
  a number of works (Mystical Theology, The Divine Names, The
  Heavenly Hierarchy, The Ecclesiastical Hierarchy) which formulated
  the basic Orthodox teaching on these subjects and inspired later
  Orthodox theologians from the time of St. Maximus the Confessor (7th
  century). Because his works seem to have been unknown in the first
  Christian centuries, and in a style and content seem to be later in
  date, it is generally accepted that in their present form they are of
  the 5th century. Modern scholars therefore call the author
  “Pseudo-Dionysius,” but in the Orthodox world, where the concept of
  “authorship” is not so restricted, there is no difficulty in seeing
  these works as in the tradition of St. Dionysius — and through him, of
  St. Paul.1


This sentiment is echoed in one of the most important Orthodox Christian theological works of the 20th century - based largely on the writings of Pseudo-Dionysius - Vladimir Lossky's The Mystical Theology of the Eastern Church:


  Modern critics, so far from agreeing as to the identity of the ‘Pseudo
  Dionysius’ and as to the date of the composition of his works, wander
  amidst the most diverse hypotheses. The way in which the critical
  researches waver between dates as far apart as the third and sixth
  centuries shows how small a measure of agreement has as yet been
  reached in regard to the origins of this mysterious work. But whatever
  the results of all this research may be, they can in no way diminish
  the theological value of the Areopagitica. From this point of view
  it matters little who their author was. What is important is the
  Church’s judgement on the contents of the work and the use which she
  has made of it. Does not the author of the Epistle to the Hebrews say
  in quoting a psalm of David: ‘But one in a certain place testified
  …’2? thus showing to what extent the question of authorship
  is of secondary importance in the case of a text inspired by the Holy
  Spirit. What is true of Holy Scripture is also true of the theological
  tradition of the Church.


So the answer I would give to your question, "Why were Pseudo-Dionysius's works accepted as authentic for so long?" is that historical accuracy was not as important issue as might be imagined in the eastern Church - with a footnote that the works are still accepted as being theologically authentic, even if  their historical origin is in question.



1 Michael Pomazanski, Orthodox Dogmatic Theology (3rd ed.), p.382
2 Hebrews 2:6