In fact, at this time the apostles were incapable of fulfilling Christ's prophecy that they would be His witnesses to the uttermost parts of the earth. The Law did not allow Jews to intermingle with Gentiles. That made it hard to be Christ's witnesses to non-Jews. The sheet out of heaven showed that the Old Testament law was no longer binding on a Christian. God was preparing Peter to go to a Gentile and preach the Gospel of Christ inside his house, probably even to the point of eating with him.
The New Covenant broke down the barrier wall of separation between the Jews and Gentiles (Ephesians 2:14). In Christ, there is neither Jew nor Gentile (Galatians 3:28). This works fine for those under the New Covenant. For those under the Old Covenant, it is a total violation of law. When Christ told them they would be His witnesses to Samaria and to the uttermost parts of the earth, He was commissioning them to a work that went outside the the boundaries of the Old Covenant.