"Arndt's fame rests on his writings. These were mainly of a mystical and devotional kind, and were inspired by St Bernard, Johannes Tauler and Thomas à Kempis.

His principal work, Wahres Christentum (book 1: 1605; books 1-4: 1606–1610) i.e. "True Christianity", which has been translated into most European languages, has served as the foundation of many books of devotion, both Roman Catholic and Protestant.

Arndt here dwells upon the mystical union between the believer and Christ, and endeavours, by drawing attention to Christ's life in His people, to correct the purely forensic side of the reformation theology, which paid almost exclusive attention to Christ's death for His people."

https://ccel.org/ccel/arndt/true/true.i.html

Johann Arndt's theological position can be summarized as follows:

Emphasis on personal piety and the inner life of the Christian: Arndt stressed the importance of a living faith, a personal relationship with Christ, and the transformation of the believer's heart. This focus on the inner life and the practical application of faith was a key element of his theology, and it foreshadowed the Pietist movement.

Union with Christ: Arndt believed that true Christianity involves a mystical union between the believer and Christ. This union goes beyond a purely legal or forensic understanding of salvation, where the focus is solely on Christ's death for our sins. Arndt emphasized the ongoing work of Christ in the believer's life, leading to sanctification and transformation.

Balance between orthodoxy and personal experience: Arndt was a Lutheran theologian who upheld the orthodox teachings of the Reformation. However, he also sought to correct an overemphasis on doctrine and forensic justification, which he felt neglected the importance of personal experience and the transformative power of the Holy Spirit.

Devotional life and practical Christianity: Arndt's most famous work, "True Christianity," is a devotional guide that encourages readers to cultivate a deeper relationship with God through prayer, meditation, and the study of Scripture. Arndt emphasized the importance of living out one's faith in daily life and serving others.

In summary, Johann Arndt's theological position can be seen as a bridge between Lutheran orthodoxy and Pietism. He maintained the core doctrines of the Reformation while also emphasizing the importance of personal piety, the inner life of the Christian, and the transformative power of the Holy Spirit. His focus on practical Christianity and the devotional life had a profound influence on generations of Christians and helped to shape the Pietist movement.