Michael Haykin:

The Bible uses two consistent images in its representation of friendship.

The first is that of the knitting of souls together.

Deuteronomy provides the earliest mention in this regard when it speaks of a ‘friend who is as your own soul’ (Deut. 13:6), that is, one who is a companion of one’s innermost thoughts and feelings.  Prominent in this reflection on friendship is the concept of intimacy.  It is well illustrated by Jonathan and David’s friendship.  For example, in 1 Samuel 18:1 we read that the ‘soul of Jonathan was knit to the soul of David, and Jonathan loved him as his own soul.’  This reflection on the meaning of friendship bears with it ideas of strong emotional attachment and loyalty.  Not surprisingly, the term ‘friend’ naturally became another name for believers or brothers and sisters in the Lord (see 3 John 14).

The second image that the Bible uses to represent friendship is the face-to-face encounter.  This is literally the image used for Moses’ relationship to God.  In the tabernacle God spoke to Moses ‘face to face, as a man speaks to his friend’ (Exod. 33:11; see also Num. 12:8).  The face-to-face image implies a conversation, a sharing of confidences and consequently a meeting of minds, goals and direction.  In the New Testament, we find a similar idea expressed in 2 John 12, where the Elder tells his readers that he wants to speak to them ‘face to face.’ One of the benefits of such face-to-face encounters between friends is the heightened insight that such times of friendship produce. As the famous saying in Proverbs 27:17 puts it, ‘Iron sharpens iron, and one man sharpens another.’

—Michael A. G. Haykin, “Spiritual Friendship as a Means of Grace,” The God Who Draws Near: An Introduction to Biblical Spirituality (Darlington, England: Evangelical Press, 2007), 73.