Here is an interesting reading and one that has been much discussed in Cambridge recently, since Luke - Acts is our first year text and an essay on Luke's view (or non-view) of the atonement is a popular one, which hinges, in part at least, on a text-critical decision about what Jesus said at the Last Supper in Luke. Here one can't get away with ridiculous generalisations ("textual criticism never affects theology").
Some years ago I wrote briefly on this, but I remain troubled by the lack of a decent explanation for the shorter reading:
Luke clearly presents the last supper (22.14-22) as a passover meal (vv. 7, 15), and describes traditional passover rituals (including two separate cups: vv. 17, 20). The interpretation of the bread and wine (in 19b, 20) is as follows:
‘This is my body which is given for you.’
‘This cup which is poured out for you is the new covenant in my blood.’ This whole passage is omitted in some manuscripts (Codex Bezae, the Old Latin and the early Syriac versions), and thus by some English translations (notably REB). As a consequence, differences of opinion concerning the authenticity of this text have resulted in different views of Luke’s theology (note to Ehrman). In this case, however, the omission is limited to only one branch of Western texts, and the vast majority of manuscripts (both early and of diverse provenance, including P75 and the major uncials) include the long version, and thus recent commentators have regarded its overwhelming attestation as ‘the decisive argument in favour of the Long Text.’
 Jeremias, Eucharistic Words, 159. The commentaries by Marshall and Fitzmyer support this; the remaining problem is to explain why the text was omitted: Jeremias suggested that the text was abbreviated in the interests of secrecy, since being a liturgical text the rest would have been well known; Metzger suggests that confusion caused by the mention of two cups led to the omission (A Textual Commentary on the Greek New Testament (London: UBS, 1975) 174).