This particular “Let us” is very appropriate to the New Testament Hebrew people because they had failed to live in accordance with it. They had trusted their special privileges and rested in them. They had become, quite frankly, lazy; they simply took things for granted.
We have much to say about this, but it is hard to explain because you are slow to learn. In fact, though by this time you ought to be teachers, you need someone to teach you the elementary truths of God’s word all over again. You need milk, not solid food! Anyone who lives on milk, being still an infant, is not acquainted with the teaching about righteousness. But solid food is for the mature, who by constant use have trained themselves to distinguish good from evil. (Hebrews 5:11–14 niv)
What the writer was saying there—bluntly—is that the Hebrews were mere spiritual infants. They had no right to be infants at that stage in their Christian progress. They had had so many opportunities over many years that by then, they should have advanced to maturity. The writer of Hebrews also explained the only way to advance to maturity. We must train ourselves to distinguish good from evil. Advancing to maturity on the path of righteousness comes in practice by training ourselves constantly. It does not happen automatically; it requires discipline. That is why one of the earlier steps was “Let us be diligent.” We must train ourselves to distinguish good from evil.
Many times, even large Christian congregations are unable to distinguish what is spiritual and scriptural from that which is just a fleshly presentation with soulish appeal. The only remedy is to train ourselves by constant use and careful practice.