All of those who have obeyed the gospel are members of the Lord's called out body - the church. None of those who have not obeyed the gospel are members of the called out body. The denominations of men do not teach the gospel plan of salvation; they delude men into thinking that they can be saved by "faith only," before and without water baptism. To speak of those who have never obeyed the gospel of Jesus Christ as being members of the church deludes people into thinking they are saved when they are lost. Those who have never obeyed the gospel are not members of the Lord's church, although they may be members of the church established by Martin Luther, John Calvin, John Smyth, Joseph Smith, and other men.
Some have suggested that New Testament Christians should fellowship all denominations. How can one become a Christian without obeying the gospel? If you do not practice and teach the doctrine of Christ, you are not are pleasing to God and fellowship (things in common) is not possible. (Eph. 5:11). Joining hands in fellowship with their violations of scripture would only endorse error and be in violation of 2 John 9-11.
The Lord's church is not made up of the good people of all denominations. It is composed of those who have obeyed the Lord's gospel in order to become a Christian and who continue to abide in Christ through faithful living. Instead of joining hands with the denominations and false teachers and their teaching, I'll make the effort to instead teach the truth and reach the lost, and ask folks to forsake humanly devised religion that will damn their souls.