Matthew 11:12 is certainly one of those verses that is considered hard to understand. I will give you my take on it. Here is the verse:
Matthew 11:12 - "And from the days of John the Baptist until now the kingdom of heaven suffereth violence, and the violent take it by force."
KINGDOM OF HEAVEN:
First, we need to understand the meaning of "the kingdom of heaven." Though I have argued that the phrase may not always be used of the exact same thing, it is often used in reference to God's visible work on earth. In some passages, it even includes professing but unredeemed members (as in the parable of the wheat and the tares). It is called the kingdom of "heaven" to indicate the origin of the work. It is God's heavenly work on earth. This meaning fits here. More specifically, it refers to the work of John the Baptist, his disciples, then Jesus, and His disciples, as they preach and offer the earthly kingdom to the Jews.
Second, who are the violent who take it by force? In a quick study, I could find no one who is called violent in a good sense in scripture--unless, as some say, this verse is an exception. God destroyed the earth in the days of Noah partly because it was "filled with violence" (Genesis 6:11). The only positive references to violence (if they can be called that) is when God brings judgment with violence (see Revelation 18:21). Other than that, the violent are always evil. This makes it difficult for me to see this verse as a description of how people come to know God--though that is a common explanation of the verse.
However, if the kingdom of heaven is God's visible working of His kingdom on earth, then it is capable of suffering violence. This brings us to the context of the verse in Matthew 11:12. Jesus had just received word from John the Baptist (who was in prison) by way of two of his disciples (Matthew 11:2). John was having second thoughts about the identity of Christ. This was evidently part of a spiritual battle John was having during his imprisonment. Jesus gave testimony of His identity to the disciples of John by way of miracles that He performed in their presence (v.4-6). Then, after these two disciples left, Jesus gave testimony of the special place of John the Baptist in the plan of God (v.7-11).
Now, who are the violent? The violent are Herod and those who worked with him to imprison John (Luke 3:19-20) and will soon behead him. They are attacking the kingdom of heaven by attacking the forerunner of the King and the first preacher who proclaimed that the kingdom of God was at hand. They took John "by force" and they will soon take the King (Jesus Christ) by force as well. The devil and the world will not win, but the kingdom of heaven will suffer much violence through their attacks. The minions of Satan are still attacking the work of God on this earth and will do so until the King comes back in power and great glory.