We may define the purity of the church as follows: the purity of the church is its degree of freedom from wrong doctrine and conduct and its degree of conformity to God’s revealed will for the church.
As we shall see in the following discussion, it is right to pray and work for the greater purity of the church. But purity cannot be our only concern or Christians would have a tendency to separate into tiny groups of very “pure” Christians and tend to exclude anyone who showed the slightest deviation in doctrine or conduct of life. Therefore, the New Testament also speaks frequently about the need to strive for the unity of the visible church. This may be defined in the following way: the unity of the church is its degree of freedom from divisions among true Christians.
The definition specifies “true Christians” because, as we saw in the previous chapter, there are those who are Christian in name only but have had no genuine experience of regeneration by the Holy Spirit. Nonetheless, many of these people take the name “Christian” and many churches that are filled with such unbelievers still call themselves Christian churches. We should not expect or work for organizational or functional unity that includes all of these people and, therefore, there will never be unity with all churches that call themselves “Christian.” But, as we shall also see in the following discussion, the New Testament certainly encourages us to work for the unity of all true believers.
From: Systematic Theology: An Introduction to Biblical Doctrine by Wayne Grudem (Grand Rapids: Zondervan Publishing House, 1994), pp. 873-874.