The law of the LORD is perfect, converting the soul: the testimony of the LORD is sure, making wise the simple. (KJV)
The first demonstration of GOD's glory was through his creation. The heavens and the firmament. Now David illustrates GOD's glory through the law. The fact that GOD's law is perfect is incredibly important. Imperfection is why man cannot enter heaven, based on his own works. With GOD everything must be perfect. The law is a witness of the LORD. And it gives the simple(the childlike, not mature) wisdom.