Some megachurches are failures at being what a church body is meant to be--a family, an organism and not an organization, and an open door to the Great Commission. If it isn't in the process of fulfilling the marching orders of the Great Commission, it is a failure. Numbers is a byproduct when we aim to do God's will, not when we specialize in gathering "crowds."
We must be able to participate and interact with fellowship and service in the body according to our gifts, and not just observe others doing the work of ministry and mission on our behalf. We are all called to be ministers of reconciliation and to contribute according to our unique gift or gifts and not be passive, but active in the church. And that implies we don't worship by proxy or vicariously.
The purpose of the church is given in the Great Commission to evangelize, equip for ministry, and build up the body of Christ to fulfill the mission to the world. The onus and responsibility for the Great Commission aren't just the pastor's but shared by all--each with a different gift, but the same Spirit. The church's job description involves discipleship, worship, and fellowship, and even one-on-one mentors.
Only when the body is functional and not ultimately dependent on any ONE gifted teacher or preacher, that even might have the charisma or be celebrated, do we have a genuine church--we don't want to build cult-like followings or personality cults, based on the teaching of one individual, no matter how gifted. Some megachurches are just crowds and you can get lost in the shuffle going there, being completely incognito or unnoticed. One should always wonder what he or she can contribute to the church's ministry to the believer or mission to the lost.
The church is to be obedient to God's Word and not try to focus on being like the world at large, or be the "church of what's happening now." There are many fads that spread throughout the body and some churches don't think they are in tune with the times if they don't follow the latest thing.
How does a church grow? By the faithful preaching of the gospel, the power is in the gospel message and it should never get old to preach or to hear. Paul strove to know nothing but Christ and Christ crucified. The lost should be able to come to the church assembly and be convicted and find out the way of salvation--this is the raison d'etre of the church!
The church's function is twofold: pastoral and prophetic to the body, while its ministry is to them, its mission is to the world. They should have a relative message to the world and an edifying message for the believer. Prophecy entails interpreting the times and making people aware of what's going on in the world, not foretelling, but forthtelling. This is because the church is the "pillar and ground of truth," according to 1 Tim. 3:15 (and all truth, according to Augustine, is God's truth), and Christians are to be the salt and light in the world, but not of it (cf. John 15:19).
Being faithful is more important in God's eyes than success in the eyes of the world; small churches can be successful because God can require some just to be faithful in little, according to their opportunities and gifts. The most important sign of a successful church is a church that is alive and vibrant and it is apparent that the Spirit is moving in the body and speaks through the preachers with edification. Jesus severely rebuked the church of Laodicea for being "lukewarm" or lackluster and lackadaisical in their worship, which was tepid and lacked spirit--We must worship "in spirit" (cf. John 4:24): "God is spirit, and those who worship Him must worship Him in spirit and in truth."
If people confess that they met God or that they sensed the moving of the Spirit during worship, God is alive in that church--there are many a moribund church that needs revival, on the other hand. If the individual members are healthy and walking with the Lord, the byproduct will be a healthy church. We want to be careful that we don't get so seeker-sensitive that people become converted to the program, instead of converted to Christ.
Don't ever lose track of keeping the main thing the main thing, our fulfillment of the Great Commission. This is why the social gospel is a misnomer, and we are not to turn stones into bread, to use metaphoric language. The church is in the business of changing lives through Jesus Christ and doing it one person at a time, and no one is beyond the reach of God's grace. It is a hospital for sinners, not a hotel for saints. and no perfect people need to apply.
Samuel said to Saul that "to obey was better than sacrifice" (cf. 1 Samuel 15:22f) and this means avoiding the Dance of the Pious or going through the motions. There are too many Christians "playing church" and even see it solely as a social event and/or opportunity to make friends. We are to obey in spirit and in truth and according to the spirit of the law, not the letter of the law, as in legalism. God is still looking for that church willing to do His will--trust and obey. This can only be done when believers realize their duty doesn't end at church attendance; which is one of the biggest misconceptions in the body. Caveat: the church should never get complacent and feel it's arrived at success--there's always room for improvement; perfection is the standard, the direction is the test.
The purpose of the church is manifold and is accomplished corporately and to be done as a family united in Christ: The mission of the Great Commission consists of a mission to the outsider or lost (evangelism), discipleship of members and attendees of the corporate body, and ministry or exercise of the gifts (including preaching or pastoral/prophetic gifts) to the church body, while observing the Great Commandment that necessitates worship of God (loving God) and mission to the lost (loving our neighbor) or what's called turning stones into bread, but not without keeping the main thing the main thing and eyes off the mission, for without vision (i.e., the mission statement or goal) the people perish (cf. Prov. 29:18). It has been well said that a great Christian has a great commitment to the Great Commission and the Great Commandment., which define the fivefold function of the church (worship, mission, discipleship, ministry, and evangelism). Soli Deo Gloria!