And he said unto them, It is not for you to know the times or the seasons, which the Father hath put in his own power.
Note 4 at Ac 1:7: The disciples had just asked a question about when Jesus would restore the kingdom to Israel (Ac 1:6). As can be seen in the actions of many in the Gospels, most people did not have the concept of a second coming of Jesus to establish God's physical kingdom on earth. They thought the first advent of the Messiah and the establishment of the kingdom would happen at the same time.
Jesus didn't directly answer their question but rather stated that the time was in the Father's control. Jesus had previously said that not even He nor the angels knew the day and the hour of this event (Mr 13:32). So, in Jesus' answer to His disciples, He spoke of knowing the time of His second coming as being beyond their control or ability to know.
However, here He revealed that with the coming of the Holy Spirit, certain things would be put under the disciples' power, and He proceeded to speak of them being His witnesses. The power to witness the Gospel to others has been delegated to us, the disciples of Jesus, and is not under God's direct control. Therefore, it is incorrect to plead with God to save someone. He has already done His part, and now we must do ours. God will not preach the Gospel for us. Not even angels are allowed to preach the Gospel for us (Ac 10:3-5). We have received the power to be witnesses. It is not by God's choosing that some have still not heard the Gospel. It is because we have not been faithful to be His witnesses.