Tertullian, unfortunately, gives us little insight into the codicology of the manuscript of Marcion's Gospel that he had at hand. Recent research in preparation for a new book (authored together with Allen Brent, Demi-monde of Late Antiquity: The social and historical phenomenon of ambiguity in Early Christian Art) has revealed that we can, indeed, find some traces not only of what Marcion's theology and liturgy looked like, but also palaeographically, how his text was written. It seems, if I am not mistaken, that he was the first who introduced nomina sacra and also the staurogram into the writing of his Gospel, hence produced, what is being seen, the earliest Christian visual art.
Recently Larry Hurtado went through the evidence of The Earliest Christian Artifacts (2008) and looked specifically at papyrological evidence to discover that the earliest forms of Christian visual representations were four nomina sacra (Theos, Kyrios, Christos, and Iēsous) and abbreviations of Jesus and Christ. The question has been raised whether or not this has to be regarded as a ‘Christian innovation’ (Hurtado) or a borrowing from Judaism (Kurt Treu, Robert Kraft), although the boundaries are blurred when Hurtado himself admits that it derives from ‘Jewish-Christian circles’. While the tachygraphic shortcut for Christ, Chi (Χ) and Rho (Ρ) had precursors in Greco-Roman times, carrying the sign of the cross (Χ), the Iota (Ι) and Eta (Η) for Jesus is otherwise unattested and may not only ‘be a Christian innovation’, but a Jewish form of gematria as attested in the Epistle of Barnabas and Clement of Alexandria. They both refer to the numerical value of Iota (Ι) and Eta (Η) which is 1 and 8, and see in it a a reference to Genesis 14:14 where the 318 servants in some of the Greek copies were written as Tau, Iota, Eta (Τ Ι Η). Barnabas and Clement ‘see in this letter compendium a foreshadowing of Jesus and his cross, Τ (= 300) = his cross, and ΙΗ (= 18) = Jesus’ name’, which equates to the Hebrew word of ‘life’ (חי) at the value of 18. In addition, we also find the combination of Iota (Ι) and Chi (Χ), another combination of Jesus and the cross. Hurtado concludes: ‘Along with the nomina sacra, the first uses of these devices, which take us back to the late second century and quite possibly earlier, represent the earliest extant expression of what we may term a Christian “visual culture”’, and, we can add, they all focus on Jesus and his cross. No surprise, then, that Hurtado adds to the nomina sacra and the shortcuts of the name of Jesus Christ the so-called Staurogram a combination of two other Greek letters, Tau (Τ) and Rho (Ρ) which are no longer shortcuts for Jesus Christ, but with the superimposing of the Greek letter Rho upon the Tau, it becomes visually and contentwise directly representative of the cross. The Staurogram is also present in very early Christian manuscripts, Papyrus Bodmer II or P66 with portions of John, dated to the end of the 2nd c., and similar in Papyrus Bodmer XIV or P75 with portions of Luke and John, as can be seen here (at the end of the fourth line):
 L.W. Hurtado, The Earliest Christian Artifacts (2006), 95-134.
 See the discussion L.W. Hurtado, The Earliest Christian Artifacts (2006), 101-34, 111. 115.
 L.W. Hurtado, The Earliest Christian Artifacts (2006), 114. 138.
 L.W. Hurtado, The Earliest Christian Artifacts (2006), 114-5; Barn. 9:7-8; Clem. Alex., Strom. VI 278-80.
 L.W. Hurtado, The Earliest Christian Artifacts (2006), 139.
‘Contrary to some widely influential assumptions’, the Staurogram is earlier than the Chi-Rho, ‘not as a freestanding symbol and general reference to Christ but in manuscripts dated as early as around 175-225 CE, where it functions as part of the abbreviation of the Greek words for “cross” and ‘crucify’, written (abbreviated) as nomina sacra … [and] a visiual reference to Jesus’ crucifixion’. We have confirmation in early Christian writings how the Staurogram was understood and even an indication by whom it was introduced. As I will show in a forthcoming paper on Marcion's early Roman liturgy, exclusively in his work Against Marcion Tertullian explains that Marcion had used in his baptismal rite the cross as the sign on the forehead, but, as he wants to show, that this should not be seen as an innovation and distinction from Judaism, but as something, already foretold by the prophet Ezekiel (9:4) and Tertullian adds:
For this same letter TAV of the Greeks, which is our T, has the appearance of the cross, which he [Ezekiel] foresaw we should have on our foreheads in the true and catholic Jerusalem.
Knowing the signification of the Tau (Τ), what, however, was the meaning of the superimposed Rho? Ephrem of Syria gives us a clue as he explains the R (Rho) of the cross in harmony with an old pagan interpretation as the sign of salvation, luck and help: ‘The R over the cross means βοήθια (help), which conforms to the value of 100.’ The Greek character R carries the value of 100 because that is the sum of the values of the characters that spell βοήθια, an example of antique isopsephy:
β ο ή θ ι α
2 + 70 + 8 + 9 + 10 + 1 = 100 = R
Already Franz-Joseph Dölger had pointed out this reference in Ephrem and Hurtado agrees – the Staurogram seems to mean ‘salvation is in the cross’ or ‘the cross is our help’. That we have nobody else in the second century referring to this interpretation and adopting it, but only Ephrem in Syria in the 4th c. could be an indication that this might have been a Marcionite notion which the copyists did not adopt, while they accepted the sign as ‘a kind of pictogram, the image of a man’s head upon a cross’, a staurogram or ‘a visual reference to the crucified Jesus’.
 L.W. Hurtado, The Earliest Christian Artifacts (2006), 136.
 Tert., Adv. Marc. III 22.
 Ephraem, In sanctam Parasceven (Ephraem Syri opera omnia quae exstant graece – syriace – latine III, Rome 1746, 477).
 F.J. Dölger, Sol (1925), 74.
 L.W. Hurtado, The Earliest Christian Artifacts (2006), 149; see F.J. Dölger, Sol (1925), 74.
 R.M. Jensen, Understanding Early Christian Art (2000), 138.
 The latter is the suggestion by L.W. Hurtado, The Earliest Christian Artifacts (2006), 151; similar already (without reference to Marcion) E. Dinkler, Signum Crucis (1967), 177-8 (referring to previous studies by Kurt Aland).