1 Corinthians 1:2 - To the church of God that is in Corinth, to those sanctified in Christ Jesus, called to be saints together with all those who in every place call upon the name of our Lord Jesus Christ, both their Lord and ours:
"The visible Church of Christ is a congregation of faithful men in which the pure Word of God is preached, and the Sacraments duly administered according to Christ's ordinances, in all those things that of necessity are requisite to the same." (from paragraph 101 of the 1952 Methodist Book of Discipline)
The classic distinction between the visible and invisible church reminds us that while God knows those who are truly his, the visible church is often found with those who genuinely know Christ and those who do not.
Even our Lord makes this distinction in the parable of the weeds (Matthew 13:24-30, 36-43). While he compares the field to the world, there are also implications suggesting the visible church will have a mixture of true and false believers. While we fallible humans will not necessarily know who is who, God will separate the true from the false at the final judgement. What is clear from this is that some are true and others are not.
In 1 Corinthians 1:2, Paul describes the church of God as those who have been sanctified (made holy) in Christ Jesus and called to be saints (called to be those who are turning from sin and pursuing a life set apart for God) with all those who in every place call upon the name of our Lord Jesus Christ.
Perhaps Paul is not telling us everything about church membership in this verse of greeting, but he is saying something important about what it means to be the church. He emphasizes that the church has been set apart to be in a relationship with Christ, called to pursue holiness, and united with all true believers who call upon the name of the Lord Jesus Christ. Here we get a glimpse of the visible and invisible church, for every local assembly contains those who are described in this verse and those who are not.
We are not called to become mind-readers and designate ourselves as those gifted with weeding out the church. But we also do not want to assume everyone who belongs to a church truly knows Christ and is pursuing holiness. The truth is, every assembly of people who make up a local church are on a spectrum, from those who walk closely with Christ to those who have not yet been born again... and everywhere else in between.
Ideally, no person who has not first been truly born again, repented of their sin, placed their trust in Christ alone for their salvation, and promised to pursue a life of holiness will be made a member of a local church. But, we know there are false professions of faith. Therefore, in light of that, here are some questions to reflect upon and talk about with others...