Even the obedience of believers has its defects
Thesis XIX. Even the obedience believers give the law has its own defects and flaws. In the very works they do in conformity with the law they fall short of the law's full exactness (ἀκρίβεια), so that in this respect there is always room left for progress, always something still to strive toward.
Paul's example makes this plain. It would be hard to find anyone to whom Christ gave so large a measure of grace, yet Paul admits he is not yet perfect and keeps reaching for what lies further on. In Philippians 3 he writes: 'Not that I have already grasped it, or am already perfect; but I press on, if somehow I may take hold of that for which Christ Jesus took hold of me. Brothers, I do not consider that I have taken hold of it. One thing I do: forgetting what lies behind and stretching out toward what lies ahead, I press on toward the goal, for the prize of God's upward call in Christ Jesus.'
Latin
XIX. Quin illud ipsum obsequium quod fideles legi exhibent suis defectibus & naevis non caret, & in illis ipsis operibus quae conformiter legi edunt summam ἀκρίβειαν, legis non assequuntur; semperque illo respectu restat iis materia profectus, & aliquid ad quod contendere debeant. Quod evidens est Pauli Apost. exemplo. Neque enim facile reperietur aliquis cui tantam gratiae mensuram indulserit Christus. Et tamen ipse fatetur se nondum esse perfectum, & se ad ulteriora perpetuo contendere. Phil. 3. Non inquit, quod jam apprehenderim, aut jam perfectus sim: sequor autem, si quo modo comprehendam, in quo & comprehensus sum a Christo Jesu. Fratres, ego me non arbitror comprehendisse. Unum autem ago, quae quidem retro sunt obliviscens, ad ea vero quae sunt priora me extendens, ad destinatum persequor, ad bravium supernae vocationis Dei in Christo Jesu.