First of all, the Lutheran Confessions begin with the Three Ecumenical Creeds, showing that the doctrine confessed in the Book of Concord is the historic Christian faith, not the opinions of a sect or cult. The Book of Concord is our treasure of God's wisdom, always relevant for Christians today, especially in these days of confusion and apostasy.
The Two Natures of Christ - Chemnitz wrote a masterpiece on this topic. The fundamental error of Calvin is his rationalistic confusion about the Two Natures. By teaching that the "finite cannot contain the Infinite," Calvin taught against the Real Presence in Holy Communion and also against the Two Natures. This rationalism erodes the doctrine of Calvin and turns it into Unitarianism.
Justification by Faith. By adding works as a requirement for salvation, the Church of Rome overturned the Gospel and turned Christ into Moses, Moses into the Savior, as Luther wrote. This semi-Pelagian heresy fuels all the errors of Romanism and creeps into Protestant theology as well. Sects often demand faith plus some work of the Law, violating justification by faith alone, apart from the works of the Law.
Efficacy of the Word versus Enthusiasm. God gave us certainty by binding His Holy Spirit exclusively to the Word. Faithfulness to God's Word is success, so no God-pleasing result can come from abandoning faithfulness to Him. Increasing the impression of success by apostasy is shameful, damaging to the ministers and their laity victims, deadly to all. In contrast, the humblest and poorest congregation is doing God's will and will bear the Fruits of the Spirit from doctrinal fidelity alone. We are not to judge results, as if we can order God around, but we must judge doctrine and practice.
The Means of Grace and Prayer. Because God has bound Himself to His Word, His Grace comes to us only through the Means of Grace - the invisible Word of teaching and preaching, the visible Word of the Sacraments. Those who want God-pleasing results in abundance only need to sow the Gospel seed in abundance. With the Internet today, this is easily (but rarely) done. The Sacraments are the best form of the Gospel for the uninitiated because they can be taught about God's will in binding His Gospel of grace to visible Means. Those who are ashamed of the Sacraments and hide them from Sunday worship are really confessing that they are ashamed of the Gospel of Christ, distrusting His efficacious Word, and substituting man's gimmicks for God's wisdom.
Prayer is the fruit of salvation, not the cause of salvation. When people are confused about the Means of Grace, they imagine they can pray Jesus into their hearts instead of realizing that Christ enters through the Gospel. They fear they must qualify with the right kind and correct fervency of prayer or they will not be forgiven. Instead, they should have the comfort of God's grace based upon the objective truth of Christ's atoning death and His resurrection from the dead.
We condemn. To continue in God's doctrine, revealed in the inerrant Scriptures, we must also condemn false doctrine whenever and wherever found. The Confessions contain many examples of false doctrine condemned in no uncertain terms. If we cannot discern false doctrine and reject it with clarity and conviction, we cannot maintain sound doctrine. Even a lowly maggot can distinguish between diseased flesh and healthy flesh. Believers should be just as discerning about doctrine, because this means life and death for souls.
