/ɪˈræstiəˌnɪzəm/
Named after Thomas Erastus, a 16th-century Swiss physician and theologian who argued that the state held authority over the church in all matters, including ecclesiastical discipline. The term now broadly refers to the doctrine of state supremacy over the church.
Erastianism is the belief that the civil government has rightful authority over the administration and discipline of the Church. This doctrine stands in direct opposition to the biblical principle of the "two kingdoms" and the headship of Christ over His Church. Scripture teaches that Christ, not the state, is the head of the church (Ephesians 5:23) and that He has delegated distinct spheres of authority to the church (for spiritual matters) and the state (for civil matters), as outlined in passages like Romans 13 and Matthew 22:21.
The term "Erastianism" is not present in Webster's 1828 dictionary. The concept is related to the definition of State: "A political body, or the whole body of people united under one government." and Church: "The collective body of Christians, or of those who profess to believe in Christ, and acknowledge him to be the Savior of mankind." The tension between these two is the subject of Erastianism.
Modern Erastianism is evident whenever the church subordinates its doctrine, discipline, or mission to the dictates or pressures of the state or prevailing political culture. This includes churches altering their teachings on marriage or ethics to conform to secular law, seeking the state's approval for their internal governance, or prioritizing nationalistic fervor over their identity as citizens of a heavenly kingdom. It is a surrender of the church's prophetic voice and its unique authority, given by Christ alone, in exchange for cultural acceptance or perceived political relevance.