Proverbs 4: 13 (NIV)
13 Hold on to instruction, do not let it go; guard it well, for it is your life.

That good thing which is committed to us we must keep, and not let it drop, through carelessness, nor suffer it to be forced from us, nor suffer ourselves to be wheedled out of it; never let go such a jewel. [Matthew Henry]

Paul likens us to shining stars, and the word shine means to reflect. The scientific term is albedo. It's a measurement of how much sunlight a celestial body reflects. The planet Venus, for example, has the highest albedo at .65. In other words, 65 percent of the light that hits Venus is reflected. Depending on where it's at in its orbit, the almost-a-planet Pluto has an albedo ranging from .49 to .66. Our night-light, the moon, has an albedo of .07. Only seven percent of sunlight is reflected, yet it lights our way on cloudless nights.

In a similar sense, each of us has a spiritual albedo. The goal? One hundred percent reflectivity. We, who with unveiled faces all reflect the Lord's glory, are being transformed into his likeness with ever-increasing glory, which comes from the Lord. You cannot produce light. You can only reflect it.

[Mark Batterson, If: Trading Your If Only Regrets for God's What If Possibilities (Baker Books, 2015), page 220]

A soldier knows that his gun is there to protect him. He is to keep it with him at all times. Likewise prayer is also our weapon to defend against evil in life. So guard what you have learned that it may protect your life.