The Church as a Means of Grace: Exclusion as DamnationIn Matthew 16 and Matthew 18 Jesus uses some odd language to describe the relationship between the actions of church leaders and "metaphysical" or heavenly reality, but the implications of what he says are important for understanding what is at stake in the actions of church leaders vis-à-vis people in their care. While I could survey the data of the many articles written over the last forty years, I think most people will find a discussion of the immediate scriptural context more enlightening and convincing as a method of revealing the meaning behind Jesus' words.
The Church as the ContextIn both cases, Jesus (or the author) makes it clear that he is talking about something that is happening in the church. In Matthew 16 Peter has confessed that he believes Jesus to be the Messiah and Jesus responds with something that would translate roughly as, "you are 'Rocky,' and on this 'bedrock' (the confession that Jesus is Messiah) I will build my church. He then segues into this seemingly unconnected section on giving the keys of the Kingdom of Heaven to Peter and says that whatever Peter limits here will be excluded from Heaven and whatever he allows will be included.
The Connection between Earth and HeavenIn Matthew 18, Jesus talks about people handling their interpersonal sins beginning with personal confrontation, escalating to bringing witnesses and culminating in a confrontation before the gathered church which, if repentance is withheld, results in exclusion. In this context, Jesus again talks about the connection between what is allowed and banned here and what is included and excluded in Heaven, speaking this time in the plural to indicate that anyone who participates in this process of facing interpersonal sin is doing something that has "metaphysical" results.
While the first text is harder to understand, the second is pretty clear: the church is a means of Grace, banning something in the church is a serious thing with consequences that can include exclusion from Heaven. I think this should be read as a warning to be careful what we decide is worthy of pursuing to the point where the church must intervene since the result can be more serious than life or death.
What about Matthew 16, is Peter special in holding the keys? I understand that if you are a Catholic, you have to say "yes," but I don't think that is what is happening here, I think Jesus is warning Peter personally that he will be in a position to allow people access to Heaven or to exclude them from Heaven. In context, Jesus makes it clear that the limit he is setting on entrance into the church is the confession that Jesus is the Christ, but that Peter will be in a position to turn away (or invite) others based on his own set of parameters.
What is God Making Clean?I think we see the reason for that warning in Acts chapter 10 , when Peter is called to the house of the Gentile, Cornelius. Peter experiences three repetitions of a vision when is is hungry of many unclean animals being offered for him and God tells him to butcher them and eat, but he refuses again and again because it is against his religious beliefs (and probably because it is disgusting). Each time God ends the vision with “What God has made clean, do not call common.” Eventually, Peter finds himself in the position of welcoming Cornelius into the church and later defending that decision against the people with whom he had agreed before his vision.
Conclusion: When is Excluding Others Christlike?If I am correct, this is what Jesus was warning Peter about, but, along with the passage in chapter 18, it serves as a warning to every Christian that the decisions we make have big consequences. If we close the doors to the church on someone who truly confesses that Jesus Christ is the Messiah, the Son of the Living God, we had better be sure that we are making the right decision because our exclusion has the potential to result in their exclusion from Heaven.
Before the vision of Acts chapter 10, Peter was pretty certain about who was banned from Heaven, but he was wrong. Could it be that you and I hold beliefs about who should be excluded that are not what God wants us to believe? Basic humility leads me to question any boundary beliefs I have, and if I have doubts, I think it far more like Christ to include those who my feelings would lead me to exclude, if I can find any reason to doubt my conclusion, than it would be for me to exclude anyone about whom I have doubts.