“They went out and preached everywhere.” This verse is more evidence that the ending of Mark is not original. The other Gospels all end before the Day of Pentecost and the start of the Christian Church. However, this verse clearly ends later in Church history. Hendrickson writes that this verse is “a statement which one would naturally associate with a period of Church history considerably later than Pentecost” (New Testament Commentary: Mark). The actual fact is that, for years after the ascension, the Jews did not catch the vision of the Great Commission as this verse seems to imply.
First and foremost, the Jews did not really start to even minister to the Gentiles until the middle of Acts. Although Peter was told to go to the house of the Gentile soldier Cornelius in Acts 10, there is no record of Jews pointedly going to the Gentiles until Acts 11:20 when Jews talked to the Gentiles in Antioch of Syria. As importantly, the Jews took a long time to go “everywhere.” The Apostles stayed in Jerusalem and outreach much beyond that was very slow at first. Although a few of the Jews who came to the feasts at Jerusalem may have believed and taken that belief home with them, one of the first major outreach events occurred when Saul was persecuting the Church and “those who had been scattered [by the persecution] went around telling the Good News” (Acts 9:4). Thus, Mark 16:20 is more like a summary verse of Acts than a conclusion of the Gospel of Mark.