Non-Christian Church Members
Not all church members are Christians! This is true in more ways than one; but because different people use these terms differently, a careful study of this matter is in order.
The church, in its universal sense, is the kingdom of Christ (COL.1:13). This refers to the relation of individuals to Christ, as subject to King. If Satan is allowed to rule in our lives, we are in Satan's kingdom (Cf MAT.12:26 JOH.8:44 ROM.6:16 etc.). Neither Satan's, nor Christ's kingdom function as a visible organized unit, but these terms depict the relation of subject to ruler in a figure.
When one becomes a Christian he becomes a citizen in Christ's kingdom, a member of His body, a branch on the vine, and a part of the called-out people, the Church (ACT.2:47 EPH.1:22-23 JOH.15:1-6 etc.). In this sense all Christians are members of Christ's church, and all true members of Christ's church are Christians. We enter this relationship by obedience to the commands of King Jesus; and those who refuse to obey Him, no matter how "good" or how many denominations they join, are neither true Christians nor members of His church.
But "church" is used in another sense -- those Christians who agree to work together as a collective unit; having Bishops (overseers) deacons (servants of the church) and pooling their means and abilities to function as one (PHI.1:1,4:15-16). This is the "local" church, dependent upon the mutual agreement of its members for its existence (see ACT.9:26-28). Let's apply our subject to this. Certainly God intended that the members of the local church be Christians. But because the human element is here involved, sometimes people are accepted as members who are not what they seem (1JO.2:19) and sometimes people truly "bought" by the blood of the Lamb (2PE.2:1) make shipwreck their faith (1TI.1:19-20). Only a few in the church at Sardis were acceptable to God (REV.3:4)
Christians are commanded to keep the local church pure. This is done by restoring the erring brother (GAL.6:1) or, failing this, to deny him their fellowship (1CO.5:1-f.)
Of paramount importance here is the fact that such breaking of fellowship, to be acceptable before God, must be done with God's word as the standard -- not majority rule (JAM.2:12,4:11-12 ROM.16:17). The church at Corinth accepted the incestuous man, but God denied him. Diotrephes led a church which cast out John's brethren, but God received them.
Yes, one may be a member of a local church, yet not be a Christian. Such a thing does not argue against the importance of the church, but it may bring shame upon the church that allows such to go uncorrected (REV.2:20). Fellowship with "church members" must never be allowed to take precedence over fellowship with the Lord.
Perhaps the greatest lesson learned here is that heaven is not gained by following a "party" of people, by stying in a certain building, or by agreeing with the majority. We must be faithful to God and His word.