Orthodoxy and Heresy - and the particular case of Marcion

Having given the AKC lectures at Kings two weeks ago, I was asked by one of the listeners in the AKC forum:

'Given that the term’s presentations are said to be about ‘orthodoxy’ and ‘heresy’, I would like to ask Dr Vinzent why he did not point out (for us beginners) that the gospel of Thomas is said to be ‘Gnostic’ or comment on what is known of Marcion’s ‘alternative’ views on God, Judaism, the Scriptures and the church in Rome at that time? Weren’t these early‘heresies’? Thank you.'

To which I replied:

I was trying to convey the process towards the distinction between 'orthodoxy' and 'heresy'. In the second century - and this is the opinio communis - this differentiation does not yet exist, neither does the category 'Gnostic'. Hence, it is not only anachronistic to call, say the Gospel of Thomas, 'Gnostic' - present scholarship is highly sceptical about this label anyway. 

The last question on Marcion is comparatively easier. Marcion was not an early heretic, even though - as Judith Lieu in her new monograph shows - he was made into one later on. During his lifetime he enjoyed the community in Rome, later formed his own community and, particularly in Rome, his community seems to have enjoyed a relationship with other Christian communities far into the third century. Nevertheless, as I have tried to show, he is also crucial for the development of 'Christianity' as a heresy within Judaism. Heresy, here and in the second century, however, should not be misunderstood. 'Heresy' at that time translated 'school opinion' and was not yet a shame label. Irenaeus of Lyon (around 180) is the first who tries to narrow the notion of 'heresy', yet still Clement of Alexandria can call himself a gnostic and people speak of their own heresies.