Proverbs 27:17 contains a proverb which describes the act of sharpening, though in the form of a metaphor. It reads:

“As iron sharpens iron, so a man sharpens the countenance of his friend.”

The Hebrew word used for “sharpen” is the word chadad, which means to "be sharp" or "make sharp." The Hebrew word translated countenance is the word paneh, or "face; the outward expression." It is the word used in Genesis 4:5-6 when Cain’s countenance, or face, fell. His expression visibly changed to one that showed displeasure. The Hebrew word used for friend in Proverbs 27:17, rea, can be translated as "darling, favorite or companion," and more than just a regular companion, it implies someone who is close.

So we can consider the Proverb to read: “As iron makes sharp iron, so a man makes sharp the face of his close companion.”

If you’ve ever had a difficult day, often when we encounter close friends whom we care about, our spirits are lifted. We are encouraged. Our face brightens up, and perhaps we are able to smile where we were struggling to smile before. In the presence of our close companions, our countenance can literally be sharpened.

So, if you look literally at the passage, that is the most likely interpretation.

Spiritual sharpening between believers

However, with all that said, this passage has often been applied to two believers sharpening each other spiritually. That, through our interactions with other believers, we can strengthen each other's faith, improve each other's character and sharpen each other. 

This concept can be gleaned from the passage. Through our interactions with others, we can sharpen one another’s outward expression (which we might consider to be our character, our habits, our attitude, or the expression of our faith). This interpretation is not outside the realm of possibility, but requires a little more reading into the passage.

So whether you take it literally, or perhaps more metaphorically, one important principle to keep in mind with this passage is that it discusses the sharpening of one’s countenance. That there is an improvement to their current state. It also specifies this interaction as occurring between close companions.

Does this passage support having theological debates?

When someone wants to get into theological debates (which can become quite heated) with another person whom they may not know very well, they will sometimes use this proverb as a defense. “Well... iron sharpens iron.” Or “sometimes, when iron is sharpening iron, there are sparks...”

Does this passage support that type of scenario?

Is a person’s countenance improved by a heated theological debate in which voices are raised and emotions flare, and both individuals leave that interaction more frustrated with one another than they started? Are you sharpening iron in that situation—or merely banging metal together?

It is an important distinction.

Whether you consider the interpretation of Proverbs 27:17 to be simply cheering up a close companion, or whether you consider it to be spiritually sharpening one another, the underlying principle is the same. It is discussing an interaction between close friends, and one or both of the individuals are leaving the interaction with their countenance, their visible outward expression, character, faith or attitude sharpened.

It is a positive interaction.

If these things are not present, or if the interaction has been overwhelmingly negative, you have not sharpened anything.

We must maintain and strengthen our relationships in the faith, and grow closer to one another to be able to create these opportunities to sharpen one another as outlined in Proverbs 27:17.