Chapter 30 of The Westminster Confession of Faith, Of Church Censures states:
1. The Lord Jesus, as king and head of his church, hath therein appointed a government in the hand of church-officers, distinct from the civil magistrate.
2. To these officers the keys of the kingdom of heaven are committed, by virtue whereof they have power respectively to retain and remit sins, to shut that kingdom against the impenitent, both by the word and censures; and to open it unto penitent sinners, by the ministry of the gospel, and by absolution from censures, as occasion shall require.
3. Church censures are necessary for the reclaiming and gaining of offending brethren; for deterring of others from the like offences; for purging out of that leaven which might infect the whole lump; for vindicating the honour of Christ, and the holy profession of the gospel; and for preventing the wrath of God, which might justly fall upon the church, if they should suffer his covenant, and the seals thereof, to be profaned by notorious and obstinate offenders.
4. For the better attaining of these ends, the officers of the church are to proceed by admonition, suspension from the sacrament of the Lord’s Supper for a season, and by excommunication from the church, according to the nature of the crime, and demerit of the person.
The following is a short excerpt from The Westminster Confession of Faith for Study Classes by G.I. Williamson, published by Presbyterian and Reformed Publishing Co., Copyright 1964, ISBN: 0-87552-538-5, regarding Ch. 30 of the WCF:
Scriptural church discipline is:
(a) not a mere matter of casting people out of the church,
nor (b) is it for “little” sin.
The purpose of church discipline is the removal of sin from the sinner, not the removal of sinners from the church. That is why excommunication is warranted only as a last resort, and then only for sin in the extreme. (Emphasis added.) Prior to excommunication, as Christ instructed (Matthew 18:15-18), repeated earnest and tender efforts are to be made to encourage an erring brother to turn from his sin. And that this process is not a mere matter of casting people out of the church is evident from the following principles taught in Matthew 18:15-18:
(1) Every church member has the right and duty to seek recovery of an erring brother. But obviously, an individual church member, who goes to a brother privately, does not seek his brother’s excommunication, but only his reformation.
(2) If possible, public knowledge of the sin in question should be avoided. Even if the first private approach should fail, the matter should still not be made public. Two or three others (presumably elders of the church) and no more should be informed of the difficulty.
(3) There is to be instruction from the Word of God in order that the erring brother may realize what the law of God requires, in the hope that he will then be persuaded to abandon his sin.
(4) Above all, it is clear that final excommunication is only a last resort. It follows every reasonable effort to reclaim the erring brother.
And therefore it may be said that excommunication requires two specific conditions:
(a) There must be undeniable violation of one of the Ten Commandments. Sin is want of conformity unto or transgression of the law of God. Mere non-conformity to custom, tradition, etc. is not punishable by discipline. (Emphasis added.) And
(b) that sin must be persisted in without repentance. It is sometimes thought that excommunication is justified only when some notorious sin such as murder or adultery is committed. The truth is that excommunication is not related to the notoriety of the sin but to the persistence of the sinner in any sin. Let the sin be slanderous gossip or neglect of divine worship, which are neither uncommon nor “notorious,” and if the sinner hardens his heart and persists in such sins without evidence of repentence, excommunication is warranted. When an erring church member has been faced with his error, when he has been shown from the Word of God what his error is and what his duty is, if he then “neglects to hear the church” (or in other words, pays no heed, but persists in his error with hardness and obstinancy), the duty of the church is clear: “let him be . . . as an heathen man and a publican.” This is right and good because Jesus said to do it.