Many Christians confuse the kingdom with the Church, and consequently, they use unscriptural phrases such as, “The kingdom of the Church.” However, the kingdom is not synonymous with the Church for the following reasons:
Firstly
The kingdom in mystery spans a greater period of time than the Church’s time on earth. It is longer in duration, having its beginning ten days before the Church began, when the Lord went back to heaven (Luke 19:12; Acts 1:9-11). And it will also continue on earth after the Church is taken to heaven (at the Rapture) to the end of the seven-year Tribulation period.
Secondly
The kingdom is broader than the Church is as far as its subjects are concerned. As we have seen, the kingdom at the present time has both “tares” (mere professors) and “wheat” (real believers), whereas the Church only consists of true believers. People may join a so-called church denomination and be on its official register, but if they are not saved through faith in Christ, they are not in the Church of God.
Thirdly
Christ is the King in His kingdom and we are His servants, but in Scripture He is never spoken of as being the King of the Church. Rather, He is Head of the Church and believers are members of His body (1 Cor. 12:12-13; Col. 1:18).
Fourthly
Matthew 16:19 tells us that Peter was given “the keys of the kingdom of the heavens.” These are not the keys of the Church, but of the kingdom. They are:
Baptism.
Discipleship.
By these two things one enters into the kingdom outwardly. But as to entrance into the Church of God—the body of Christ—a person must be born of God and sealed with the Holy Spirit (John 3:5; 1 Cor. 12:12-13; Eph. 1:13).
Fifthly
In the fellowship of the Church, we are to put away leaven by excommunicating the person or persons in whom it is found (1 Cor. 5:11-13). In the kingdom of the heavens (in mystery), evildoers and the leaven are not removed, but are permitted to go on "until the harvest" (Matt. 13:28-30).