Christ’s Church is Christian, because its only foundation is Christ and the apostolic-prophetic word of the Bible, which He has given.
1 Cor. 3:11: For other foundation can no man lay that is laid, which is Jesus Christ.
Eph. 2:19-20: Ye are fellow citizens with the saints, and of the household of God, and are built upon the foundation of the Apostles and Prophets, Jesus Christ himself being the chief Cornerstone.