Having considered godliness as a major element in Venning's theology, we come now to its antithesis, sin. Indeed, an ‘overwhelming awareness’ of sin was characteristic of puritanism, and struggle against the world, flesh and devil central to the puritan experience. It is certainly a major element in Venning's theology, yet, pertinently, it also reflects his emphasis upon the pursuit of happiness.

Venning discusses sin extensively in Sin, The Plague of Plagues; Or, Sinful Sin the Worst of Evils (1669). His purpose is five-fold, amongst which are the proselytising aims, ‘For admiring the free and rich grace of God’ and for ‘believing in our Lord Jesus Christ’. In Venning's view, recognition of the ‘sinfulness of sin’ is essential to the ordo salutis and being spiritually ‘awakened’. He therefore promotes ‘hating of, repenting for and from sin, thereby taking a holy, just and good revenge on it and our selves’ and ‘lastly, that this black spot may serve for a set off, to the admirable, incomparable, and transcendent Beauty of Holiness’. The latter theme, a puritan re-interpretation of the language of Laudian piety, was sufficiently important to Venning to warrant a treatise of its own.

Venning adopts the rhetorical imagery and language of a courtroom, intent on proving the utter sinfulness of sin. He cites witnesses against it, including God, Christ, Angels – holy, fallen and the devil; men – both saints and sinners; Creation; the Law; and the Gospel. Moreover, sin is a witness against itself. While he acknowledges that a number have expounded the nature of sin, he claims to have been distinctive: