To sharpen means to remove roughness and unevenness from a blade or a point to improve its ability to cut or pierce. For one object to sharpen another, there must be very precise alignment and movement in their relative positions over time; one must be positioned to receive the sharpening influence and the other must be positioned to provide it, and the hands of an experienced craftsman must bring the two together repeatedly. It does not happen all at once, but over many repeated, precisely engineered strokes.
God employs this as a fascinating illustration of how friends edify one another: “Iron sharpeneth iron; so a man sharpeneth the countenance of his friend.” (Pr 27:17) When friends deliberately invite each other to facilitate their pursuit of holiness, God can use them to this end with miraculous precision.
As the countenance is the reflection of the heart, this sharpening is the refinement of disposition and orientation, the honing of character. It is not merely teaching, or even discipleship or modeling, but a purification, a transformation, something only God can do.
Christ the Master Craftsman (He 3:4), Who indwells the individual believer (1Jn 4:15), the communion of believers (2Co_6:16), and works in and through all men (Pr 16:1), can sharpen us through anyone if we’ll position ourselves to hear Him and submit ourselves to Him. But it is in building relationships particularly for this purpose, cultivating the trust of godly friends, where we can be more open to seeing His way through the loving challenges of those who know us well (Pr 27:6), that His finest work is done.