The most important statement for the Old Catholic Church is the Declaration of Utrecht, issued in 1889. 

This document serves as the foundational charter of Old Catholic doctrine and polity, clearly stating their position against certain Roman Catholic dogmas that they view as innovations contrary to the ancient faith of the Church. 

Key points from the Declaration of Utrecht include:

• Adherence to the ancient Catholic faith: Old Catholics accept the ecumenical creeds (Apostles’ and Nicene) and the dogmatic decisions of the first seven ecumenical councils of the undivided Church.
• Rejection of Papal Infallibility and Universal Jurisdiction: The Declaration explicitly repudiates the First Vatican Council’s (1870) decrees concerning the Pope’s infallibility and his supreme, universal authority over all churches.
• Affirmation of the Vincentian Canon: The statement upholds the principle laid down by St. Vincent of Lérins: “We hold that which has been believed everywhere, always, and of all men; for that is truly and properly Catholic” (Id teneamus, quod ubique, quod semper, quod ab omnibus creditum est; hoc est etenim vere proprieque catholicum).
• Episcopal-Synodal Governance: Authority resides in the collective body of bishops (collegiality), with significant participation of both clergy and laity in local synods, rather than a single central figure.
• Focus on Christian Unity: The Declaration expresses a hope and commitment for the reunion of all Christian confessions based on the shared faith of the ancient Church. 

The Declaration of Utrecht is fundamental because it defines the Old Catholic identity, which seeks to preserve the core Catholic tradition while rejecting later additions to dogma and centralizing hierarchical structures.