As the 1950's and 60's famed band, Kingston Trio, sang in their ballad, "Reverend Mr. Black":

You got to walk that lonesome valley.

You got to walk it by yourself.

Oh nobody else can walk it for you.

You got to walk it by yourself.

Your pastor cannot go with you. Your friend cannot step up and vouch for you. Not even your spouse can stand beside you on that day. You will stand before the glorious and terrible King of Kings by yourself and give an account for the deeds you have done (Rom 14:12). If you had trusted Christ as your liberation and righteousness then every sin from your life on earth will record like a black hole or a blank space of loss, but no shame apart from a sense of loss need be suffered (1 Cor 3:15), for Christ suffered the guilt and shame already on the tree at Calvary. You will enter into the presence of Jesus with joy that defies all description (Mat 25:21). 

Yet even though you and I stand accountable to God for each of our actions (Ecc 12:14), as individual as these actions may be, they are never completely individual. Because you were not just saved from sin but also saved into the body of Christ, you cannot sin or do good without it effecting the body of Christ which is the believers, the universal church. 

Even though a single man stole treasures from the plunder of conquered Jericho, God still moved the author of Joshua to write, "But the sons of Israel acted unfaithfully in regard to the things under the ban..." (Jos 7:1). One man's sin represented the entire community of Israel. In the next battle, the corpses of 36 Israelite warriors would make this point (Jos 7:3-5). 

We are free to make choices by our own decision, but these choices are never completely individual. Not only do they effect others (whether others realize the effect or not and whether they know what we did or not), but they are influenced by the company we keep. It's impossible to say that one made his decision completely free of influence. We humans are incredibly vulnerable to influence and even though I may choose all by myself to buy a house or sell a car, many people over the years of my life had a part in that decision. One man made me see the value of estate investment, another showed me why selling a car before its value plummets can be financial wisdom. I made a decision, but that decision was not free of influence, and therefore, my decision was not entirely individual. 

This is why community matters so much. We need people. Christians to strengthen us, unbelievers to keep eternity in our face so that we don't take for granted the eternal life so freely showered on us through Jesus. Christians make us strong, unbelievers keep us humble and real.