Let's look at what Matthew records. In Matthew 18 Jesus is speaking of the process of church discipline (this is another answer to be covered later as to why we need the local church.) However, prior to that, in Matthew 16:18-19 Jesus lays out for Peter how he will be used in establishing the Church:
"And I tell you, you are Peter, and on this rock I will build my church, and the gates of hell shall not prevail against it. I will give you the keys of the kingdom of heaven..."It is clear from what Jesus says here that there is a direct connection between his "Church" and the kingdom of heaven. (And by the way, we have record of Jesus saying a lot about his kingdom.) The terminology "kingdom of heaven" appears 31 times in the book of Matthew. At the beginning of his ministry he is warning people to repent because the Kingdom of Heaven is at hand. To be a follower of Christ means that you have an inheritance in his kingdom, and in turn you are a part of his global Church. So we cannot dismiss the importance of the Church because we don't have record of Jesus saying the word a lot.
So what is the connection between the global Church and that group that meets in that building down the street from you? We see the beginning of the global Church in the book of Acts. In short: the Holy Spirit comes upon the Apostles, Peter preaches, people are saved, the Church is started. Later in Acts 20:28 Paul is giving instructions to the elders of the church at Ephesus.
"Pay careful attention to yourselves and to all the flock, in which the Holy Spirit has made you overseers, to care for the church of God, which he obtained with his own blood."It is clear from this verse that the church is comprised of those whom Jesus bought with his blood, thus they have been redeemed. So is Paul instructing the elders in Ephesus to pay careful attention to all believers everywhere? No, they are to pay careful attention to the believers whom the Holy Spirit has made them overseers. This is a subgroup of the whole group of global believers. They are to care for the church of God at Ephesus. This is very clear from the context of this passage. This is just one example from scripture, but we can see throughout scripture that the local church in a specific location is to be seen as a part of the global Church representing the kingdom of God. Anyone who has an inheritance in the kingdom of heaven should be associated with and a part of the church. The local church is supposed to be representing the kingdom here and now, and those who are a part of the eternal kingdom are to be its representatives.
I will close with an illustration. Think of it this way, the local church is an embassy for the kingdom of heaven located on foreign soil. This world is corrupt and fallen, but the church is a safe haven for those whose citizenship is ultimately in heaven. Its purpose is to do the work of the kingdom. In the church the citizens come together and reflect on their true identity. The local church is the venue for the praise of their king. It is the place where the citizens can learn more about their heritage, their king, and their traditions. The local church represents their Lord to the host nation by being a local embassy for the kingdom of heaven. A Christian who does not participate with a local church is like an ambassador who does not use their embassy.
"Therefore, we are ambassadors for Christ, God making his appeal through us. We implore you on behalf of Christ, be reconciled to God." - 2 Cor 5:20