How did the rise of the heretical sect known as ?Gnosticism? actually lead the Church

How did the rise of the heretical sect known as ?Gnosticism? actually lead the Church to clarify the canon of?

Gnostics were the original Christians. If anyone is the heretical sect it was the Paulians.

Your q isn't complete - what canon are you referring to?

Before any canon of the Roman Catholic Church was established, the Gnostic teacher, Marcion, established *his* canon of what he considered to be the only authentic scriptures.

There are those who believe that Marcion's establishing the *first* Christian canon motivated another Christian sect of the time (that developed into the RCC) to re actively establish their *own* canon, since they viewed Marcion's teachings as being "heretical". So they established as "canonical" the works that fit *their* beliefs & supported what *they* were teaching.

http://www.earlychristianwritings.com/marcion.html

But Marcion's was hardly the *only* "Gnostic" sect at that time! Afterall, Simon Magus *may* have been born before Jesus was -- & was supposedly the head of John the Baptist's religious movement, after Dositheos.

To refer to "the" heretical sect known as "Gnosticism" is to be under-informed & to be making a very *false* generalization. Actually there were dozens of *different* sects of early Christians that are currently lumped under the broad category of "Gnosticism". The Valentinians were no more the *same* "Gnostic" sect as Marcion's sect than were the Simonians, the Sethians, the Barbelognostics, the followers of Bardesanes or the Carpocrations (& several more that i'm not listing).

To be believing these different "Gnostic" groups were one *single* sect of "Gnosticism" is equivalent to believing that Roman Catholics are the same as Mormons, Jehovah Witnesses, Seventh Day Adventists, Lutherans & Baptists! There were Gnostics that weren't even *Christians* (such as those who reverenced the teachings in "Thrice Greatest Hermes" -- & perhaps many of the followers of Simon Magus). To be confusing the followers of the Christian Gnostic, Marcion, with those who revered "Thrice Greatest Hermes" would be pretty equivalent to confusing a Roman Catholic believer with a practitioner of Hinduism who reverences Krishna, Shiva, Shakti, & Kali!

It appears that the Gnostic, Cerinthus, preceded Marcion -- perhaps actually in the *presence* of the author of the first two epistles of John!

http://en.wikipedia.org/wiki/Cerinthus

The two links i've provided in Source(s give detailed answers to your Question -- from the conservative (non-Gnostic) Christian perspective.