John Calvin Commentary Numbers 15:38

"Speak unto the children of Israel, and bid them that they make them fringes in the borders of their garments throughout their generations, and that they put upon the fringe of each border a cord of blue:" — Numbers 15:38 (ASV)

Speak unto the children of Israel. I will explain the purpose of this precept more fully a little later, although it is clear from the next verse that God had no other aim than to train the Jews in constant meditation on the Law.

For there was no religious power contained in the fringes themselves, nor did that material texture have any inherent value. But since people are lazy and forgetful in cultivating piety, God intended this aid to provide for their weakness.

For when He says that they should look upon it and remember, He implies that they need these simple aids, which can appeal even to their outward senses. He also implies that unless their memory was kept alert, it was very likely that forgetfulness would creep up on them. But He then adds that God is not satisfied with mere empty knowledge; He demands sincere devotion and practical action.

In the latter part of the verse, He points out another requirement, namely, not only that their sluggishness should be spurred, but also that their waywardness should be restrained. For when He says, that you seek not after your own heart, He suggests that unless God restrained their wandering inclinations, they would be greatly inclined to all kinds of superstitions and errors.

And, first of all, by contrasting the hearts and eyes of people with His Law, He shows that He desires His people to be content with that one rule He prescribes, without mixing in any of their own inventions. Again, He denounces the futility of whatever people devise for themselves; however pleasing any human plan may seem to them, He still repudiates and condemns it.

This is expressed even more clearly in the final phrase, when He says that people go a whoring whenever they are governed by their own plans. This statement deserves our special attention, for while those who worship God according to their own will feel much self-satisfaction, and while they consider their zeal to be very good and very right, they do nothing else but pollute themselves with spiritual adultery.

For what the world considers to be the holiest devotion, God, with His own mouth, pronounces to be fornication. By the word “eyes,” He undoubtedly means humanity’s power of discernment.