Absolute perfection and comparative perfection
Thesis XXI. To resolve this difficulty we need a twofold distinction of perfection. One kind is called perfection absolutely; the other is perfection only in a certain respect, by comparison. What is absolutely perfect lacks nothing in its kind. Things are called perfect by comparison when, though far short of the highest perfection, they still surpass many other things of the same kind with which they are compared.
When we say that no one in this life is perfect, that is, no one gives God's will a perfect obedience, we mean absolute perfection: perfection that has reached the top and can receive no addition. In this sense Paul admits he is not yet perfect, since he had not yet reached the degree of love and obedience toward God to which the blessed spirits reigning with Christ have been raised. Their knowledge, and their obedience and love as well, can grow no further, because they are carried toward God with all their powers and the whole affection of their will.
None of this stops believers who outstrip others in virtue, and who have advanced further in knowing divine truth and in striving after piety and righteousness, from being called perfect. They are perfect not simply, but relative to those who fall well behind them here: those who are still, in Paul's words, infants in Christ and in some way carnal, compared with others who are called spiritual in a special sense (see 1 Corinthians 3). These are, so to speak, the grown-ups among Christians, of a riper age and stature, if one may put it so. They are the ones Paul calls perfect in the passages cited, when he says, 'We speak wisdom among the perfect,' and, 'Let as many of us as are perfect think this way.'
Latin
XXI. Quae difficultas ut solvatur observanda est duplex perfectionis distinctio. Est enim perfectio quaedam absolute sic dicta, alia vero secundum quid & comparate tantum. Absolute perfectum est illud cui in suo genere nihil deest. Comparate vero perfecta dicuntur ea quae licet a summa perfectione absint, multa tamen superant alia ejusdem generis, cum quibus instituitur eorum comparatio. Quum dicimus in hac vita nullos esse perfectos, sive qui perfectum divinae voluntati obsequium praebeant, istud intelligimus de perfectione absoluta, quae pervenit ad summum, & ad quam nihil addi potest, quo sensu Paulus agnoscit se nondum esse perfectum, quia nondum pervenerat ad illum gradum amoris & obsequii erga Deum, ad quem evecti sunt beati spiritus cum Christo regnantes; quorum non tantum cognitio, sed obsequium etiam & amor, nullum amplius potest sumere incrementum, cum totis omnino viribus, totoque voluntatis affectu ferantur in Deum. Verum hoc non obstat quo minus fideles qui virtute caeteris praecellunt, & qui in veritatis divinae notitia, & pietatis ac justitiae studio majores progressus fecerunt, perfecti nominentur, non quidem simpliciter, sed respectu eorum qui multum illis hac in parte cedunt, quique sunt adhuc, ut Paulus loquitur, infantes in Christo, & quodammodo carnales, prae illis qui singulari ratione spirituales appellantur, ut videre est 1 Cor. 3. Et hi sunt inter Christianos, ut sic dicam, adultiores, & aetatis ac staturae, si ita loqui licet, magis provectae, quos Paulus locis ante citatis perfectos vocat, dum ait, Sapientiam loquimur inter perfectos, &, Quicunque perfecti sumus hoc sentiamus.