Every good work must be referred to God as its end
And as on his view every good work must proceed from a sincere love of God, so too it must be referred to God as its end. However good what someone does may be in its substance, unless he does it for God and with regard to God, he cannot avoid sin in it.
He sets out to prove this in chapter 17 of the third book of On the state of fallen nature, often cited, where his thesis is: 'No good work at all, that is, no work without sin, can be done unless it is referred to the end to which true wisdom commands that it be referred, that is, to God loved for his own sake. Otherwise it is neither done rightly nor a good work, but is polluted by the contamination of true sin.'
Hence also in that book and the next he everywhere draws the necessity of faith for acting well from this: faith is what directs the intention in acting toward that end.
Latin
XXXVII. Atque ut omne bonum opus, juxta ipsum, ex sincero Dei amore procedere debet, ita quoque illud ad Deum ut finem referri necesse est. Quantumcunque vero quod aliquis agit sit, secundum substantiam, bonum, nisi faciat illud propter Deum & Dei respectu, in eo peccatum vitare non potest. Illud probare contendit capite decimo septimo libri tertii saepius allegati de statu naturae lapsae, ubi ejus pronunciatum est, Nullum omnino fieri posse opus bonum, hoc est, sine peccato, nisi ad finem in quem referri vera sapientia praecipit, hoc est in Deum propter se dilectum, referatur: alioquin nec recte fieri, neque bonum opus esse, sed veri peccati contaminatione pollutum. Unde quoque in eo libro & sequenti passim arcessit fidei necessitatem ad bene operandum, quoniam ipsa est quae ad finem illum intentionem in agendo dirigit.