from Prescription Against Heretics
Chapter I.-Introductory. Heresies Must Exist, and Even Abound; They are a Probation to Faith.
 The character of the times in which we live is such as to call forth from us even this admonition, that we ought not to be astonished at the heresies (which abound)2 neither ought their existence to surprise us, for it was foretold that they should come to pass;3 nor the fact that they subvert the faith of some, for their final cause is, by affording a trial to faith, to give it also the opportunity of being "approved."4  Groundless, therefore, and inconsiderate is the offence of the many5 who are scandalized by the very fact that heresies prevail to such a degree. How great (might their offence have been) if they had not existed.6  When it has been determined that a thing must by all means be, it receives the (final) cause for which it has its being. This secures the power through which it exists, in such a way that it is impossible for it not to have existence.
Interesting and arresting words for our time. Rather like--"There will always be wars and rumors of war." So too, "Heretics will always be among you--they try the word and find error so that those who stay loyal to the path can know the truth."
Don't know about the reasoning, but at least an interesting document from the Tertullian project.