That the liberty of believing or of not believing is placed in free choice
In Deuteronomy: “Lo, I have set before your face life and death, good and evil. Choose for yourself life, that you may live.” Also in Isaiah: “And if you be willing, and hear me, you shall eat the good of the land. But if you be unwilling, and will not hear me, the sword shall consume you. For the mouth of the Lord has spoken these things.” Also in the Gospel according to Luke: “The kingdom of God is within you.”
Source: The Treatises of Cyprian (New Advent)