1) SCRIPTURE REQUIRES FULL DOCTRINAL AGREEMENT - Excerpt
There is nothing in the Scriptures to suggest that the unity of faith which is required as the basis for the outward expression of church fellowship is limited to agreement only in the doctrine of justification or a few fundamental doctrines. It is true that many of the doctrinal disputes referred to in the New Testament involved fundamental doctrines. When he wrote Galatians, Paul was battling a denial of the doctrine of justification by grace. In his epistles John appears to be battling a heresy that denied Jesus’ humanity. But many other types of doctrinal error are specifically referred to in the New Testament, including denial of the resurrection of the body (2 Timothy 2:18), teaching Christians that they could disregard God’s commandments since the forgiveness of sins was free (Rev. 2 & 3, Jude 3-10, 2 Peter 2:1-4, 13-20), forbidding marriage and prohibiting certain foods (1 Timothy 4:3), and quarrels about genealogies and the law (Titus 3:9). This list is comprehensive enough to demonstrate that the apostles’ concern for doctrinal purity was not limited to a few key doctrines.

COMMENTS: WELS Q & A

2) SPIRIT TO SPIRIT
Outward fellowship is not enough, spirit must fellowship spirit.

SOURCE: Lenski

COMMENT: Fellowship in spirit, doesn't mean that two people get along, but what they believe is the same. They may have some difference of opinion as to adiaphora and how to do things, but they are in agreement with the Spirit, faithfully submitting to all that the Bible says.