(Part 1) I have been reflecting on this question over the past few days and ended up writing and then redrafting a response several times. I do not know what your understanding of the field is, but this is a difficult question to answer. Almost everything in early Christianity is up for debate. There are major fault lines over the belief structure, forms of worship, and community organisation for the church in general, let alone when the question of ‘heresy’ is introduced.

A safe answer might suggest this as a picture: for most Christians, their religion consistent of regularly meeting up with each other to pray, sing, reflect on the scriptures, and conduct rituals such as the eucharistic meal and baptism. By scriptures this would mostly have meant the Jewish Biblical texts, though by the second century this started to include contemporary texts such as the letters of Paul, the Gospels, and other expressly Christian texts and by regularly meeting I mean that they could be meeting daily, or even multiple times a day. Location-wise, they would meet up in places that were not obvious to the public eye, so this would include the houses of members of the community, rented halls, or outdoor locations when it was dark (early morning or late at night). There would have probably been a few dozen Christians per city on average and so had some sort of organisational structure that included a recognised leadership. This could have taken the form of a council of elders or a singular bishop with authority over a number of community groups. They would have prayed regularly, often using the Lord’s prayer but also others from the scriptures or their own extemporary prayer.

To my mind, a Christian Gnostic would likely have done all of the above but with a few changes. I would suggest that Christian Gnostics (I will expand on this concept later) were actually embedded within the main Christian community and they would have seen themselves as true Christians. The amin differences would be that they would probably have had additional meetings with other like-minded individuals to discuss special Gnostic interpretations and they would probably have been more suspicious of the Jewish elements of Christianity and so may not have taken part in the Lords prayer. Instead they may have focused more on prayer directed at Jesus instead but beyond that I think their lives would have been more or less indistinguishable from other Christians. In fact, as I mentioned, they would have seen themselves as Christians and likely have been in communion with the wider community. The idea of some kind of ‘secret knowledge’ (Gnostic means ‘knower’ and refers to secret knowledge relating to salvation) is something that they may have kept hidden, aware that many Christians did not approve, or in some cases may have been open about it and formed parallel or rival communities but ones that would have been substantially similar to other Christian ones.

The reason for this answer is because I am increasingly leaning towards the view that ‘orthodoxy’ and ‘heresy’, in the sense of ‘correct’ and ‘incorrect belief’ were not clearly defined positions until probably quite late in the second century, or even later than that. I think what probably existed was an extended community of believers who had varying interpretations and over the years their debates shaped narratives that set boundaries on what types of divergence were acceptable and what types were not. A lot of the second century heresiologists who have been termed forerunners of the orthodox position, or ‘proto-orthodoxy’, write against Gnostic Christians, but these enemies seem to be involved with their churches. They must have been close enough to the church to have been seen as threats and the descriptions we have of early heretics like Valentinus and Marcion show that they are all initially members of the church. On this basis it makes sense that many of their structures and practices were the same although over time they may have diverged. Indeed, Marcion (not a Gnostic admittedly but a second century heretic so a decent example) is excommunicated from the church in Rome and goes on to form his own Marcionite Church, the structures of which include leadership by bishops, the baptism of members, and a New Testament Canon. Where they differ is in who is allowed to be baptised and what texts are included in their Bible.