6 But, beloved brethren, this is so, because faith is lacking, because no one believes that the things which God promises are true, although He is true, whose word to believers is eternal and unchangeable. If a grave and praiseworthy man should promise you anything, you would assuredly have faith in the promiser, and would not think that you should be cheated and deceived by him whom you knew to be steadfast in his words and his deeds. Now God is speaking with you; and do you faithlessly waver in your unbelieving mind? God promises to you, on your departure from this world, immortality and eternity; and do you doubt? This is not to know God at all; this is to offend Christ, the Teacher of believers, with the sin of incredulity; this is for one established in the Church not to have faith in the house of faith.
Source: The Treatises of Cyprian (New Advent)