Hence, the appearance of John the Baptist signaled the transition to the Messianic era, the time when God’s Kingdom would advance much faster and further than it had ever done before, the time when the Gentiles would come into the Kingdom in great numbers.
I do not think that it refers to the violence suffered by the OT prophets, because the wording of v.12 indicates that the violence begins from the days of John the Baptist. Rather it seems to indicate violence that begins after the time of the OT prophets, i.e. in the times of John and Baptist and Jesus Christ.
Who then are the ‘violent’ who take it by force? One possible interpretation is that spiritual warfare really intensified during the time of John the Baptist and of Christ (before He resurrected). That may explain the many instances of demonic possession recorded in the gospels when Christ had to cast out demons. Satan was hard at work, trying his best to prevent Christ from carrying out His work and ministry, and attempting to snatch the kingdom of heaven by force. But his efforts failed when Christ died and rose again, and all power (authority) was given to Christ in heaven and in earth (Matthew 28:18). Satan still works today, but as a defeated enemy, who has received the deathblow at the cross.
Another possible interpretation is to take the ‘violent’ as men, e.g. the Pharisees, Saducees, etc. who opposed Jesus (who represents the kingdom of heaven). Hence their efforts to oppose, arrest and crucify Christ would constitute the violence suffered by the kingdom of Heaven. John the Baptist himself (who also represents the kingdom of heaven) was arrested and executed by a violent man, king Herod.
Yet another possible interpretation is that the violent are the common people who when they saw John the Baptist and Jesus, and the miracles performed, got excited and thought that the political messianic kingdom was about to begin, and so they wanted to launch a violent rebellion, overthrow the present government and crown Christ as king of Israel. John 6:14,15 - "Then those men, when they had seen the miracle that Jesus did, said, This is of a truth that prophet that should come into the world. When Jesus therefore perceived that they would come and take him by force, to make him a king, He departed again into a mountain himself alone." (cf. Acts 1:6).
The truth is that the kingdom of heaven is spiritual, not a political one yet. It is a kingdom of Christ dwelling and ruling in the hearts of men. Violence is not the means that God will use to establish His kingdom (as Liberation theology advocates). Christ explained the true nature of the Kingdom of heaven in the parables of the kingdom (Matthew 13) in order to correct this prevalent misconception.
It is hard to say which of these views is the right one. We will know the answer one day, when we see Christ!