Challies has a great post on his sight about the communal nature of growing in holiness (a spiritual process for which the big word is "sanctification"). That is, growing in Christ-likeness - growing in holiness - takes place most efficiently in the context of relationships with other people. This is a rather foreign idea to us in the West (and it even grates on me a bit as I write it here, in fact). It's something that doesn't come naturally to us. Why not? Because we are independent people. There's nothing standing in the way of what we want to achieve other than ourselves. We can do anything we put our minds to. We live in a very individualistic society, so when we talk about other people being instrumental to our spiritual growth process, it's not an easy thing to consider.
Earlier this summer I preached a three week series on the biblical concept of fellowship. One of the key points I tried to make in those sermons was that fellowship - and all of the benefits of fellowship - always happen in the context of relationships. One cannot have Christian fellowship with oneself. It just doesn't work that way. And the Bible is clear that fellowship with other believers is a necessity for Christians. You can't be a Christian and live outside of Christian fellowship.
Challies quotes a speaker he recently heard who had this helpful thought regarding spiritual growth in the context of a community: "Sin doesn't do well in the light." The point here is that when our sins are exposed to other people we are held accountable for our actions, and can therefore repair the damage done to our relationships (temporal and spiritual) when our sin is exposed. The key here is that you have to be in community with other people in order for your sin to be exposed. You don't expose your sin to yourself - you already know what it is. The Holy Spirit does indeed expose sin to the believer, but it's difficult to be held accountable to righteousness and to mortifying those sins on our own. This is why we need other people "doing" physical and spiritual life alongside us: because sin doesn't do well in the light. The more we have people examining us in light of scripture (and vice versa, of course), and the more we have people holding us accountable to what we've put forward in those relationships, the more power we will have in conquering that sin.
Challies makes another great point that I had pondered before, but hadn't thought of in the same way he presents it. He argues that my spiritual growth is not only good for me, but it's also good for the community - the church; the group of people I'm doing life with. Why is it important to people in my community that I am growing spiritually? Because the more I am able to conquer sin, the more I can help them do the same; the more I grow, the more I can encourage others in their growth; the more I come to serve God and others, the more they can do so as well.
When you boil it down, it's almost like a a symbiotic relationship. The more I grow, the more you grow. The more you grow, the more I grow, and so on and so forth. Living in community with other people is not just a primer for spiritual growth, it is the lifeblood.