Repent and Believe in the Gospel!
Before Jesus, to convert always meant a "going back" (the Hebrew term, "shur," means to reverse course, to go back on one's steps). It indicated the act of the one who, at a certain point of life, realized that he was "not on course"; then he paused, reconsidered; decided on a change of attitude and returned to observance of the law and the Covenant with God. He made a real change of direction, a "U-turn." Conversion, in this case, has a moral meaning; it consists of changing customs, of reforming one's life.
This meaning changes on Jesus' lips. To convert no longer means to go back to the ancient Covenant and observance of the law; rather, it means to make a great leap forward and to enter the Kingdom, to cling to the salvation which has come to men gratuitously, by the free and sovereign initiative of God.
Conversion and salvation have exchanged places. There no longer is, as before, the conversion of man and therefore salvation as God's recompense; rather, salvation is first, as generous and gratuitous offer of God, and then conversion as man's response. In this consist the "glad tidings," the joyful character of evangelical conversion. God does not wait for man to make the first step, to change his life, to do good works, almost as if salvation is compensation for his efforts. No; grace precedes, it is God's initiative. In this, Christianity is distinguished from all other religions: it does not begin with preaching duty but gift; it does not begin with the law, but with grace.
"Repent and believe": This phrase does not mean therefore two different and successive things, but the same fundamental action: Convert, that is, believe! By believing, be converted. Faith is the door through which one enters the Kingdom. If it had been said: The door is innocence, the door is exact observance of all the commandments, the door is patience, purity, one might say: it's not for me; I'm not innocent, I am lacking in this or that virtue. But we are told: The door is faith. It is not impossible for anyone to believe, because God has created us free and intelligent precisely to make the act of faith in him possible for us.