Unionism is the practice of participating in fellowship with those with whom there is not unity in Christian doctrine. It is a false show of unity with those with whom there is no agreement concerning the teaching of the Gospel or the administration of the sacraments (cf. AC VII); with those with whom there is not unity in teaching and in all articles of the faith (cf. SD X.31).
Unionism takes place because of the doctrinal indifference which prevails not just in Christianity in general, but also in the LCMS and LCC in particular. Doctrinal agreement is seen either as impossible, or as unimportant.
As Francis Pieper points out, unionism is contrary to Christ’s warning to beware of false prophets (Matt. 7:15) and Paul’s admonition to “avoid them” (Rom. 16:17). It ignores John’s instructions, “If anyone comes to you and does not bring this teaching, do not receive him into your house or give him any greeting, for whoever greets him takes part in his wicked works.” (II John 10-11)
Scripture clearly teaches that doctrinal agreement is important, and necessary for fellowship. To be indifferent to doctrinal differences and participate in a unionistic service with those who are false teachers is to take part in their wicked works and be a partaker in the sins of others (I Tim. 5:22).
To say that it is impossible to have doctrinal unity is to say that Scripture is unclear. It is to show that one is uncertain as to what is truth and what is error. As Pieper also points out, “indifferentism is the signature of uncertainty. One who is certain of the truth is never indifferent to doctrine, but insists uncompromisingly on pure doctrine, even as Scripture, both in the Old and in the New Testament, insistently demands that the doctrine be ‘pure,’ uncontaminated by any human Ego product.” (Christian Dogmatics I, 118)
This is not to deny that there are Christians in heterodox church bodies. Rather, it is to stand firm on the truth of God’s Word so that those Christians would also hear the truth in all matters of doctrine.
Unionism claims to unify Christians, but the opposite is true. True unity can exist in the church only if it is united in the truth. Worship together in services that are watered down theologically so that no one is offended is the key result of unionism. The truth is pushed far away from such false fellowship, causing further division.
There is one Lord, one faith, one Baptism (Eph. 4:5) and we are appealed to, in the name of our Lord Jesus Christ, that all of us would agree, and that there be no divisions among us, but that we be united in the same mind and the same judgment (I Cor. 1:10). Where there is disagreement in doctrine, where the one Baptism instituted by Christ is rejected as the work of man, where there is a different mind and different judgment, there is not unity in faith.
For the sake of our fellow Christians in different church bodies, let us hold fast to our confession, without wavering, knowing that the truth sets us free. Let us gladly talk about our differences in the light of what God’s Word says, instead of sweeping them under the table and participating in false fellowship. Let us strive for true unity in the church, which is unity in doctrine. This is worked towards through a diligent use of the means of grace, and a serious study of the Word of God and the Confessions to which our pastors and churches have subscribed.