Iron sharpens iron, so one man sharpens another. Proverbs 27:17
A good surgeon will use only the sharpest of knives. Why? It's because the sharper the knife, the cleaner the cut it makes. And the cleaner the cut, the closer the two sides match each other. The closer the two sides match each other, the easier it is for them to come together again in the healing process. Dull knives and chain saws make for hack jobs; the ripping up of flesh so that the two sides don't match each other. Healing is made much more difficult if not impossible. The word of God is spoken of as sharper than any two edged sword.
The irony of many Christians who hold to "doctrinal purity," and require the strictest adherence to doctrinal minutiae for fellowship, church membership, ministry leadership or pastoral candidacy is that they don't understand how iron sharpens iron. I can't tell you how many times I've heard Ephesians 4:3, "being diligent to preserve the unity of the Spirit in the bond of peace", interpreted as, "you must agree with every doctrinal position of the elders/church/by-laws, etc." Disagreement in any form is shunned.
But iron sharpens iron because each piece is rough. They grate against each other and the friction caused by the imperfections is what eliminates the imperfections, resulting in a sharp knife or sword. When surgery is needed, success results. When roughness in beliefs is disallowed, and everybody in a particular group is required to believe the same exact thing, no sharpening can occur. The result is, ironically, the exact thing that is not allowed. What the doctrinal purists who distance themselves from others who disagree are in effect saying, is, "I will not be sharpened by another piece of iron." And so they exist as drawers full of dull, rusty knives. When surgery is needed, a hack job results, and the one in need of surgery is not healed, but rather wounded even more.