Moses is clear: the Torah was doable. It was not in heaven; it was not in the abyss; it was very near to each Israelite.
Yet there are people calling themselves Bible-believing who insist instead that the Law was impossible. They are fond of quoting verses out of context: “There is none righteous, no, not one,” says Romans 3:10, they will tell you.
What they neglect to tell you is that Romans 3:10 is a quotation from Psalm 14:3, and that if we turn to that Psalm — for the NT always assumes that you know the context of the quotation; there are no “prooftexts” in it — lo and behold! We find that two verses below it, David says that “God is present in the company of the righteous.” Is David speaking hypothetically? Or what?
What does it mean to be “blameless”? The word is not actually all that common. The Hebrew in the OT is naqah. It occurs 44 times, and seems generally to mean “free from guilt” or from judicial responsibility for an action. Often this is a matter of someone else taking responsibility for it:
2Sa 3:28 And afterward when David heard [it], he said, I and my kingdom [are] guiltless before the LORD forever from the blood of Abner the son of Ner… [Implies that Joab is not.]
Or even more obvious, 2Sa 14:9 And the woman of Tekoah said unto the king, My lord, O king, the iniquity [be] on me, and on my father’s house and the king and his throne [be] guiltless.
When we combine this insight with Leithart’s insights on the nature of “imputation” in the Bible — that the assigning of guilt for a sin is distinct from the sin itself — we can see that it is a very different matter to say that someone is “sinless” than it is to say that they are “blameless.”
The Bible never calls anyone except Jesus “sinless.” But it calls many people blameless: Job, Zacharias and Elizabeth, etc. But it is especially interesting that Paul refers to his preconversion self as both “a persecutor and a violent man” and as “blameless, as touching the righteousness that is in the Torah.”
Paul means that he, in his life as a Pharisaic Jew had been, not sinless, but faithful to avail himself of the means of expiation that God had provided in the Torah. Likewise the parents of John the Baptizer.