In a letter to Pope Pius IX, Charles Hodge explained why American Presbyterians would not be sending delegates to the First Vatican Council in 1869. In this letter, Hodge gives an incredibly winsome, succinct, and helpful articulation of why Protestants in general are not heretics or schismatics (contrary to the claims of Roman Catholics, et al).
At the heart of the letter - and the heart of the matter - Hodge explains the frequently misunderstood truth about Christian unity:
We cordially recognize as members of Christ's visible Church on earth, all those who profess the true religion together with their children. We are not only willing but earnest to hold Christian communion with them, provided they do not require, as conditions of such communion, that we profess doctrines which the Word of God condemns, or that we should do what the Word forbids. If in any case any Church prescribes such unscriptural terms of fellowship, the error and the fault is with that church and not with us.It is vital to remember that the guilt of division lay upon those who turn away from the truth of Christ revealed in Scripture, not upon those who refuse to discard this truth for the sake of visible or organizational coherence.
Read the full text of Hodge's letter at Banner of Truth.