Apologists argue that original witnesses to Jesus life and ministry were still alive at the time the gospels were written. They say this is important because the influence of the eyewitnesses would have served as a check on fabrications. I have previously questioned the logic of this argument in On the Lack of Debunking and Further Thoughts on the Lack of Debunking. However, after reading the Epistle of Clement to the Corinthians, another possibility occurs to me.
Maybe Clement's declined to cite Mark and Luke not because he did not know of them, but because he knew that they were not authetic. Maybe he did not cite them because he knew that they did not accurately reflect what he had learned from Peter and Paul. Maybe it was the influence of the original disciples that delayed the general acceptance of the gospels until they had all been gone for almost a century.