George Ladd, in The Gospel of the Kingdom, writes about the mystery of the Kingdom of God (mystery meaning that something was hidden for a time is now revealed):
This is the mystery of the Kingdom: that the Kingdom of God has come among men and yet men can reject it. The Kingdom will not experience uniform success. Not all will receive it. This was a staggering thing to one who knew only the Old Testament. When God’s Kingdom comes, it will come with power. Who can resist it? Who can withstand God? But precisely this is the mystery of the Kingdom. The Kingdom is here, but it can be rejected. One day God will indeed manifest HIs mighty power to purge the earth of wickedness, sin and evil; but not now. God’s Kingdom is working among men, but God will not compel them to bow before it. They must receive it; the response must come from a willing heart and a submissive will.
God is still dealing with us in this same way. God will not drive you into His Kingdom. It is not the business of those who are called to the ministry of the Word to speak with authoritarian compulsion. We speak as emissaries of God, but we plead and do not demand, we persuade and do not drive. We implore men to open their hearts that the Word of His Kingdom may have its fruitage in their lives. But men can reject it. They can spurn the Gospel of the Kingdom. They can scorn the preacher of the Word; and he is helpless.
– George Eldon Ladd, The Gospel of the Kingdom, pp. 56-57.