One of the things that Marcion's "unique" view of scripture force the Church to do was to come to an agreement about the accepted Christian writings.
When the early Christians spoke of "scripture," they were referring mostly to the Hebrew Scriptures, namely, the Septuagint (the Greek version of the Old Testament). In their gatherings, they also read passages from different Gospels, as well as several letters from Paul. Marcion, however, caused the Church to respond to the issue of accepted writings in a more organized way.
Most people have the misconception that the canon was decided on in a very formal matter. This is not the case. The Hebrew scripture was already accepted, so it was the New Testament books that needed to be decided on. The consensus over the NT books developed gradually over a period of time, until the Church came to a general agreement about which books would be included.
The four Gospels were the first books to gain popularity and acceptance. But why four and why not only one? Indeed, there are slight differences in each, so why not just avoid the problem and use only one? The early Christians were aware of this, and it is precisely why the used more than one. Many Gnostic's taught that the heavenly messenger had trusted his secret knowledge to a certain disciple, who alone became the one true interpreter of the message. For example, some Gnostics had the Gospel of Thomas and Marcion used Luke. Therefore, the Church's use of four Gospels was a direct challenge to Marcion and Gnosticism. In essence, the Church was stating that her teachings were not based on just one, single witness, or one single apostle, but upon the entire apostolic tradition and teaching.
By the end of the second century, the "core" of the canon had been basically agreed upon, with the book of Acts and Paul's letters earning early recognition. The rest of the books were soon to follow, but it wasn't until the second half of the fourth century that complete consensus was achieved in regards to the New Testament (for more on the canon, you can listen to our podcast here)..
The Church also responded by writing what we now call the "Apostles' Creed." There is much mystery and fiction that surrounds how this creed came about, but it was more than likely put together in Rome, around 150 A.D. The purpose of the creed was to be able to distinguish the true believers from heretics. Those who could confirm this creed proved that they were neither Marcionites nor Gnostics (a separate post on the Apostles ' Creed is forthcoming, in which I will deal with the specifics of how it refutes these heresies).
Finally, the Church also responded with an appeal to Apostolic succesion. Marcion and the Gnostics argued that they had "secret" knowledge that was delivered by Jesus, the heavenly messenger. Through a sucession of secret teachers, they gained acess to this secret knowledge.
The Church argued that if Jesus really had some secret knolwedge to give, he would have shared it with his disciples, but he did not. In essence, the same people that Jesus entrusted the building and founding of the Church should have been the recipients of this secret knowledge as well. Thus, they would have then turned around and taught it to others. Therefore, any secret teaching should be found among the apostles and their disciples, but it is not. All those who were alive during the second century and could claim to know the apostles personally denied that such a knowledge was taught or ever existed