This is a common theme that Protestants raise, in attempting to make the point that we are all fallen to such an extent that no one can ever even possibly rise above sin. Catholics agree that no one can save themselves (over against the heresy of Pelagianism). But we’re told that no one ever has, or could, perfectly keep the Mosaic law (with its 613 commandments). Protestants then assert that this was why we needed grace. Again, Catholics have no objection to that broad statement; we concur that salvation ultimately comes by God’s grace alone. It’s not a disagreement between us, and never has been.

Protestants will often cite James 2:10 in this regard: “whoever keeps the whole law but fails in one point has become guilty of all of it” (cf. Galatians 3:10: “all who rely on works of the law are under a curse”). Looked at closely, however, James wasn’t asserting in the first place that no one can ever perfectly keep the law. His point was that failing in one point is the same as failing in all. Logically, that’s a different concept. Protestants who bring this up simply assume from the outset that no one can do it, which is invalid circular reasoning.

On a Calvinist site, I found the article, “Of Man’s Inability to Keep the Law Perfectly” written by a Scottish Presbyterian theologian, Thomas Boston (1676-1732). He wrote:

There is not a just man upon earth. By the just man in this text is not meant an evangelically just man, or one just in respect of parts, though not of degrees; but one who is legally so, just in the eye of the law, as having yielded perfect obedience to all its commands ...

There can be no perfect keeping of the commands of God …

Since Adam fell, no mere man is able, while in this life, either of himself, or by virtue of any grace now given, to keep the commands perfectly. Of himself he cannot do it; neither is there any measure of grace given to any in this life, whereby they may be enabled to do it ... And there is no promise of grace given in the word, whereby believers may be enabled to keep the commands of God perfectly …

The Psalmist, however, assumes the opposite, and refers to “those whose way is blameless, who walk in the law of the LORD!” (Psalm 119:1) and presupposes the possibility of complete adherence: “May my heart be blameless in thy statutes” (Psalm 119:80). St. Luke, writing in his inspired Gospel, makes the same point about Zechariah and Elizabeth, the parents of John the Baptist (see Luke 1:5):

‘And they were both righteous before God, walking in all the commandments and ordinances of the Lord blameless.’

And that’s not all. We also have the corroborating testimony of no less than the great St. Paul, who described his former life as a zealous follower of Judaism: “as to righteousness under the law blameless” (cf. Galatians 1:14). 

Maybe someone at this point would raise a question about the meaning of the Greek word translated “blameless.” It’s the same word in both Luke 1:6 and Philippians 3:6: amemptos (ἄμεμπτος).

Strong’s Lexicon on this same page defines it as “a state of being without fault or blame. It conveys the idea of moral integrity and uprightness, often in the context of one’s conduct before God and others. In the New Testament, it is used to describe individuals who live in a manner that is above reproach, adhering to God’s commandments and maintaining a pure and righteous lifestyle.”

Thayer’s Greek Lexicon gives the meaning for these two passages as “blameless, deserving no censure ... free from fault or defect.” Baptist Greek linguist A. T. Robertson, in his Word Pictures in the New Testament, commenting on Philippians 3:6, states that it indicates that Paul “knew and practised all the rules of the rabbis. A marvellous record, scoring a hundred in Judaism.” So he kept all the laws!

God the Father is repeatedly presented in the Bible as assuming that his people were capable of keeping his laws and (synonyms) statutes and ordinances and covenant and his commandments: given to Moses on Mount Sinai. Otherwise, it was all a big farce: He commanded them to do something that he knew all along was impossible.

But the Bible informs us that the Psalmist kept the law (Psalm 119:56, 67, 69, 100, 168) and that he verified that others did so as well (119:63). So did King David, according to God:

• … David my servant whom I chose, who kept my commandments and my statutes (1 Kings 11:34)
• ... you have not been like my servant David, who kept my commandments, and followed me with all his heart, doing only that which was right in my eyes (1 Kings 14:8)

And Moses, Aaron, and Samuel (see Psalm 99:6):

• ... they kept his testimonies, and the statutes that he gave them.

And the children of Levi:

• ... they observed thy word, and kept thy covenant. (Deuteronomy 33:9)

And King Hezekiah:

And he did what was right in the eyes of the LORD, according to all that David his father had done. ... He trusted in the LORD the God of Israel; so that there was none like him among all the kings of Judah after him, nor among those who were before him. For he held fast to the LORD; he did not depart from following him, but kept the commandments which the LORD commanded Moses. And the LORD was with him; wherever he went forth, he prospered. (2 Kings 18:3, 5-7)

Thus, our choice is God’s inspired, inerrant revelation or unbiblical traditions of men originating 15 centuries after Christ.