Bad trees cannot bear good fruit
Thesis I. Christ says more than once that a bad tree cannot produce good fruit. By nature every human being is a bad tree: we are conceived and born in sin, and, as God himself testifies through Moses in Genesis 6, the human heart's every feeling and thought leans toward evil from childhood.
So if people are left to themselves, no work can come from them that is truly good, pleasing to God, and of any value for salvation. Whatever they think or do without God's grace, by natural strength alone, may look splendid from outside, but there is always something rotten inside it, and that makes it simply faulty.
Latin
THESIS I. CHRISTUS non uno in loco pronunciat malam arborem non posse facere bonos fructus. Porro homines universi per naturam arbores malae sunt. Quippe in peccato nati sunt & concepti, & sensus ac cogitatio cordis humani ad malum prona est a pueritia, ut Deus ipse apud Mosem testatur cap. 6. Gen. Ac proinde ab hominibus, si sibi ipsis relinquantur, nullum opus procedere potest vere bonum & Deo gratum, & quod sit ad salutem alicujus momenti: sed quicquid homines absque Dei gratia, & solius naturae viribus cogitant, aut faciunt, quantumcunque exterius splendere videatur, semper tamen intus aliquid putre habet, unde simpliciter vitiosum redditur.