Was Judaism already declining from the time the Law was given to Moses?
Yes. From the Law given to Moses around 1500 B.C., Judaism as a religion was in a constant state of degeneration, moving downward rather than staying at perfection, until by Christ's earthly ministry it was a dead, man-made religion with no power of God left in it.
Les explains that from the giving of the Law to Moses about 1500 B.C., Judaism was in a constant state of degeneration. He compares this to entropy: things seldom stay at a peak of perfection and always generate downward. By the time of Christ's earthly ministry, it was nothing but a dead, man-made religion, with no power of God left in it. Les notes that even Israel's religion came up into God's nostrils as a stench, citing Isaiah chapter 1 starting at verse 11, and warns that many church services can be the same.