When the words immediately following this are placed alongside Mark 16:9-20, the correspondences in language are quite striking:

Having therefore received their orders, and being fully assured by the resurrection [αναστασεως] of our Lord Jesus [κυριος Ιησους] Christ, and full of faith in the word [τω λογω] of God, with full assurance of the Holy Spirit they went out [εξηλθον] proclaiming the good news [ευαγγελιζομενοι] that the kingdom of God was about to come . . . preaching [κηρυσσοντες] in the country and in the towns . . . (1 Clem. 42.3-4)

Having been raised [αναστας] . . . he appeared to the Eleven . . . and he said to them, “Go into all the world and preach [κηρυξατε] the gospel [το ευαγγελιον] to all creation . . . “ So then, after the Lord Jesus [κυριος Ιησους] had spoken to them, he was taken up into heaven, and sat down at the right hand of God. And going out [εξελτοντες] they preached [εκηρυξαν] everywhere, the Lord working with them and confirming the word [τον λογον] through the accompanying signs. (Mark 16:9, 14-15, 19-20)

First to be noted is the identify of setting. Like Mark 16 the passage from Clement deals with post-resurrection events, specifically with the commissioning of the apostles and their going forth to preach the gospel. There is then obvious thematic coherence. In this context unique to that ending among all the Gospel accounts. Regarding the apostles going out to preach, the particular verb chosen by Clement to describe that event (εκελθειν) is the same as that occurring in Mark 16:20 of precisely the same action. None of the other Gospel writers uses this verb in this context. The uniqueness with respect to the verb found in the Markan ending makes a strong connection between Clement an that intertext. The verb “preach” in the active voice with the apostles as grammatical subject appears in both Clement (κηρυσσοντες) and the disputed verses of Mark (κηρυξατε, εκηρυξαν), yet not in this particular way in any of the other Gospel endings. Luke is the only one here to employ the same verb, though evidently in quite a different manner. Luke makes no explicit mention of the apostles as the agents of preaching, while his use of the verb is passive with the abstract noun “repentance” as the grammatical subject. Moreover, Clement and Mark are further united in using “preach” absolutely, that is, without an explicit grammatical object. The former has the phrase “preaching [κηρυσσοντες] in the country and in the towns,” and the latter “they preached [εκηρυξαν] everywhere.” In each instance the absolute verb is qualified by a locative expression. Undoubtedly there is much semantic overlap between “in the country and in the towns” and “everywhere.” Indeed, it may be the case that, for stylistic reasons, Clement here consciously avoided using “everywhere” (πανταχου) since he had used this very term just a few sentences before in 41.2. Whether this is so or not, there is a specific semantic and structural correspondence at this point between the two phrases which is unparalleled in the Gospels. Also found in both writers is the definite noun “the word” referring to the message preached. The sense of GK is another uniquely Markan feature in the Gospel endings. The presence of all these elements together in a passage relating an identical setting, plus the fact that the other Gospel endings do not contain such usages, makes not merely a good case but an extremely forceful one for Clement’s familiarity with the questioned ending of Mark. If so, the significance of this cannot be overestimated since Clement’s letter is generally dated to the late first century.