On the mountain, Jesus "called to Him those He Himself wanted" (Mark 3:13).
Presumably there were more people following Jesus than the five handpicked men Mark mentions. Out of that group, He called Peter, Andrew, James, John, Matthew, and others. He called them not to a study of the law, of science, or of a trade; rather, He called them to Himself. Jesus called the ones He wanted, and His call was a sovereign one, because everyone He called to that office came to that office, and they came willingly to join that band of men who were to be a part of whom He was.
In a sense, this is a microcosmic look at what Jesus does for the whole kingdom of God—He calls those whom He wants. The Greek word that is translated as "church" in the Bible is ekklesia. This word is made up of a prefix and a root. The prefix is ek or ex, which means "out of " or "from." The root word is a form of the verb kaleo, which means "to call." Thus, ekklesia means "those who are the called-out ones." Simply put, the invisible church, the true church, is composed of those who are called by God not only outwardly but inwardly by the Holy Spirit. When Jesus calls someone to discipleship, He is calling that person to Himself, to belong to Him, to follow Him, and to learn from Him and of Him.
It is true that the only faith by which a person can be justified is his own faith. No one can be justified by his spouse's faith, his parents' faith, his children's faith, or anyone else's faith. At the final judgment, everyone will stand before God alone, and judgment will be rendered based on what is in his heart alone.
However, every time Christ saves an individual, He places him in a group. There is a corporate dimension to the kingdom of God that we must not overlook. I spoke recently with a woman whose church has called a new pastor. She is not happy with the new pastor, so she has left the church. When I asked her what she is doing for worship, she replied that she watches religious programming on television on Sunday morning. The obvious problem with this is that she is not in church on Sunday morning. She is not with the people of God in corporate worship, in solemn assembly. The Christian life is a corporate thing, for Christ places His redeemed people in the church to learn together, grow together, serve together, and worship together.
Excerpt from R.C. Sproul's commentary on Mark.