Answer: Because going to church is a visible, tangible expression of our love and worship toward God. It is where we can gather with other believers to publicly bear witness of our faith and trust in God, and it is where we can bring Him offerings of praise, thanks, and honor (see Psalm 22:22)

The following "Question" was asked of John MacArthur Jr., the pastor of Grace Community Church, Sun Valley, California. 
Copyright 2001 by John MacArthur Jr., All Rights Reserved.

Answer:

The New Testament repeatedly emphasizes the importance of local assemblies. In fact, it was the pattern of Paul's ministry to establish local congregations in the cities where he preached the gospel. Hebrews 10:24-25 commands every believer to be a part of such a local body and reveals why this is necessary. 

"And let us consider how to stimulate one another to love and good deeds, not forsaking our own assembling together, as is the habit of some, but encouraging one another; and all the more, as you see the day drawing near" (Hebrews 10:24-25).

It is only in the local body to which one is committed that there can be the level of intimacy that is required for carefully stimulating fellow-believers "to love and good deeds." And it is only in this setting that we can encourage one another. 

The New Testament also teaches that every believer is to be under the protection and nurture of the leadership of the local church. These godly men can shepherd the believer by encouraging, admonishing, and teaching. Hebrews 13:7 and 17 help us to understand that God has graciously granted accountability to us through godly leadership.

Furthermore, when Paul gave Timothy special instructions about the public meetings, he said "Until I come, give attention to the public reading of Scripture, to exhortation and teaching" (1 Timothy 4:13). Part of the emphasis in public worship includes these three things: hearing the Word, being called to obedience and action through exhortation, and teaching. It is only in the context of the local assembly that these things can most effectively take place.

Acts 2:42 shows us what the early church did when they met together: "They were continually devoting themselves to the apostles' teaching and to fellowship, to the breaking of bread and to prayer." They learned God's Word and the implications of it in their lives; they joined to carry out acts of love and service to one another; they commemorated the Lord's death and resurrection through the breaking of bread; and they prayed. Of course, we can do these things individually, but God has called us into His body-the church is the local representation of that worldwide-body-and we should gladly minister and be ministered to among God's people.

Active local church membership is imperative to living a life without compromise. It is only through the ministry of the local church that a believer can receive the kind of teaching, accountability, and encouragement that is necessary for him to stand firm in his convictions. God has ordained that the church provide the kind of environment where an uncompromising life can thrive.

• God commands us to meet together as the Church. Our purpose is to worship God and encourage one another. Hebrews 10:25

• The Church is the Family of God. Many people think of the Church as a building, but it is really the people inside of the structure. To be a Christian means you are part of the Family of God. Other Christians are your new brothers and sisters in the faith. Jesus established a "Community of Believers". 

• Jesus loves the Church and died for the Church. Ephesians 5:25

• The Church and Jesus are inseparable!  In Ephesians 5:23 the Bible says Christ is the Head of the Church, which is His Body. 

• In Acts 2:47 the Bible says that when you become a Christian, you become a part of the Church.