Biblically, grace seems to be a proactive, transformative reality. It is a gift, but it is a gift that, once received, must be reflected and modeled to the rest of the world. As Scot McKnight suggests in his book, Embracing Grace: A Gospel for All of Us, once we embrace grace, we are to become an "embracing" people who exhibit grace toward the other...precisely because it is unmerited. It is less a benefit for members of a club...and more a new reality that they have bought into. To accept it for myself and refuse to extend it to others betrays both the gift of grace and the Giver. Don't just take my word for it, Jesus actually has quite a bit to say about this in the gospels (i.e. his comments on forgiveness after "The Lord's Prayer" and the Parable of the Unforgiving Servant...just for starters)
It's usually at this point that the fear impulse kicks in. What if we're wrong? This is the fear that haunts those raised in our tradition. Let me clear it up for you. We are. We have 3 pound brains and we dare to speak of an infinite God. When asked if he would fellowship a "brother in error", Fred McClure used to routinely respond "I don't have any other kind. We're all in error on something." N.T. Wright regularly begins his lectures by saying something to the effect of "I'm wrong about roughly 1/3 of what I'm telling you. I just have no idea which 1/3 that is." And thank God for His grace that washes over sin and "error" as he is actively working to form me into the image of His son... into the future where His dream for me and the world are reality.
The Bible is quite clear that the measure of judgment we apply to others will be applied to us. While I highly doubt that the Bible's concept of judgment is as equivalent to the American judicial system as we tend to presuppose in these discussions, the implications are hard to miss. We are going to be wrong about some things, but if, when it comes to judgment, "the measure I use" will be "measured to me", I want to "err" on the side of grace.