Exegesis:

humeis ‘you’ is emphatic: Translator’s New Testament ‘You, who do you say…?’ (cf. The Modern Speech New Testament ‘You yourselves…’; Manson, Zürcher Bibel, Le Nouveau Testament. Version Synodale, O Novo Testamento de Nosso Senhor Jesus Cristo. Revisdo Autorizada).

apokritheis … legei (cf. 3.33) ‘answering … he says’: ‘he answered.’

ho christos (cf. 1.1) ‘the Anointed One,’ ‘the Messiah,’ ‘the Christ’: here employed as a title, not as a proper name. Most English translations have ‘the Christ’ (cf. Zürcher Bibel der Christus, Lagrange le Christ, O Novo Testamento de Nosso Senhor Jesus Cristo. Revisdo Autorizada o Cristo); Manson and Translator’s New Testament, however, ‘the Messiah.’

Translation:

Who do you say that I am is as complex as the parallel question in verse 27. In Tzeltal this may be expressed as ‘Who am I, if you should say?’

In general the Christ is translated in a form more or less equivalent to a proper name, even though in this instance it is a title. The reasons for this are (1) any expression such as ‘the anointed’ often involves cultural connotations which are strange and hence difficult to explain, and (2) any such terms as ‘the appointed one’ or ‘the chosen one’ must usually be expanded to indicate by whom, e.g. ‘the appointed one by God.’ Accordingly, a transliterated form of Christ is preferred.

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .