Seriously though probably the Ebionites or the Marcionites, both throw interesting light on Christianity's first century. The former still followed the Law, championed James and reviled Paul; the latter were Paul stans who went the opposite way and rejected the Old Testament, were the first group to collect the letters of Paul and carried around a Gospel some think was the basis for gLuke.
Yes, stylistic similarity is the strongest argument in favour of a joint Luke-Acts predating anything Marcion had. But that depends on an analysis of the final version of the Gospel of Luke that we have in the New Testament, which obviously cannot prove authorship of an earlier version - this is the question. Henry Cadbury, the 20th century's leading expert on the language of Luke cautioned this:
Such stylistic analyses, [Cadbury] maintains, can only reach conclusions about the final editor of a text, who is able to put his own distinctive linguistic and stylistic varnish over the entire work, “whatever the underlying sources or development” of earlier editions.
Scholars suspect that for example, the first two chapters of Luke containing the Nativity of Jesus were added later, because chapter 3 begins as does the opening of Mark, which Luke obviously knew and was copying. The genealogy of Jesus too is awkwardly inserted into Luke 3 after the baptism. Jesus' agony and sweating of blood in the garden of Gethsemane also doesn't appear in the earliest manuscripts we have of Luke, nor does the institution of the Eucharist in Luke 22:19-20. These are just examples. The problem here is that Tertullian's attack on Marcion claims that Marcion's gospel
deleted passages from Luke, and didn't contain a birth narrative! So if one thinks as many do that e.g. Luke's Nativity was tacked on later, this is an argument for Marcion's gospel preserving an earlier version.
