Matt. 24:14 reads, "And this gospel of the kingdom will be preached in the whole world as a testimony to all nations, and then the end will come." The prophecies recorded in Matthew 24 are an answer to the questions the disciples privately asked Jesus. These disciples were Jesus' followers and they had heard Jesus preach the kingdom many times. They were the only ones who were granted understanding of the God's plan and who could, therefore, testify to the nations.
The Jews, who were unbelieving, did not understand the gospel of the kingdom. Recall Matt. 13:10, 11, 15, "The disciples came to him and asked, 'Why do you speak to the people in parables?' He replied, 'Because the knowledge of the secret of the kingdom of heaven has been given to you, but not to them…For this people's heart has become calloused;…otherwise they might see with their eyes, hear with their ears, understand with their hearts and turn, and I would heal them.'" Jesus was not trying to convert the Jewish nation at His first advent. The conversion of Israel will occur at His second advent when God again fights for the nation as He did in Old Testament times. Zechariah 12:9, 10, "On that day I will set out to destroy all the nations that attack Jerusalem. And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and supplication. They will look on me, the one they have pierced, and they will mourn for him as one mourn for an only child, and grieve bitterly for him as one grieves for a firstborn son." (See also Zech. 14:3.)
The Bible has already been given as a witness to all nations. It has been translated into hundreds of languages. Additionally, the stories of Christian martyrs and their testimonies are known in most societies. This verse does not state how the nations will receive these truths. It does not say the nations will be converted. In fact, Psalm 2 tells us the nations will be angry when Jesus returns. "Why do the heathen rage, and the people image a vain thing? The kings of the earth set themselves, and the rulers take counsel together, against the LORD, and against his anointed, saying, Let us break their bands asunder, and cast away their cords from us." (Psa.2:1-3)