Proverbs 27:17
Iron sharpeneth iron; so a man sharpeneth the countenance of his friend.
Suppose you go to the store and buy a knife. It could be smaller than your finger, or it could be an 18" machete. You can use it as it comes, or you can put a sharp edge on it. Regardless of what you do, as you use it it loses its edge. When it does, you must sharpen it. The key to making most knives sharp, is to hold the blade at the same angle to the stone on both sides. If you don't use the same angle, it will not be as sharp as it could be.
People are the same way. Living life tends to make us lose our "edge", until we are not as effective as we once were. We have a couple things to sharpen ourselves with. First, is the Bible. We also have other Christians. The problem with using other Christians, is that sometimes they don't use the "angle" on both sides. If we rub someone the wrong way, we could drive them away from church. (Something we may have to answer for one day.) So, we must speak the truth in love, or we will be like a clanging brass. No one wants to listen to that...
Hebrews 10:23-25
Let us hold fast the profession of [our] faith without wavering; (for he [is] faithful that promised;) And let us consider one another to provoke unto love and to good works: Not forsaking the assembling of ourselves together, as the manner of some [is]; but exhorting [one another]: and so much the more, as ye see the day approaching.