A non-Catholic person suggested to me that the early ecumenical councils of the Church were not authoritative since they weren’t even initiated by The Church, but rather by the Roman emperors. The suggestion here was that political power was used to justify theology, doctrine instead of relying on the power of the Holy Spirit. I didn’t have a good way to answer this.
How do we defend the legitimacy of these councils if they weren’t even convened by the Church?
Is this person a Christian? If he is, then he must come to terms with the fact that the Trinity, Scripture, the Hypostatic Union, etc., all developed from the approval of the community of Christians, which is a nice way of saying the Bishops of the Church guiding the Baptized (back then, the fact that most Protestants didn’t have Bishops defending their views would meanthat Christians would ignore them. Anyone could claim to be representing the real Church, but only those with an ordination going back to the Apostles held any authority. At least the Arians had many of the ordained on their side!

).
If he isn’t, then I can see how he would believe that the Councils are just political power grabs: without faith (or at least the consideration of faith), your friend literally cannot see the difference between the a Holy Spirit and practical politics. The distinction is possible, and reasonable, considering the basics of the Christian Faith, but to outright deny these principles
a priori leaves someone with really no other choice other than to see the Councils as mere power struggles. In this case, you must address to reasonability of Christianity itself first.
You first have to ask your friend: what
is an Eccumentical council? And I mean “what is it most essentially?”
A Catholic would answer (this is a simplification) that an Eccumentical council is a gathering of Bishops, who are clarifying dogma and doctrine, which are eventually approved by the Head of the Church (aka the Bishop of Rome aka the Pope). This is what a council is, period. How the Bishops ended up together doing this or whatever else is “by happening” or accidental to what a Eccumentical council is.
Charles V basically organized Trent, St. Cyril sneakily planned Ephesus so that less representatives of Antioch would be there, and St. Nicolaus took a wack at Arius at Nicea. Do any of these deny the essential definition of a Council? Does the fact that Nicea was conducted in Greek do anything to make a Council not a Council? If we assume that Nicea was a valid council, would Vatican II not be a council because the Fathers of Nicea traveled to Nicea on horses, while the Vatican II Fathers traveled in planes? Of course not! All these things are merely accidental to what a Council
is. The language of the council members, the nationality of most of the members, the hairstyle, the mode of transportation, the political forces that gathered those Bishops together, and even the Heresy that the Council is responding to are all irrelevant to what a Council
is, in a Catholic’s eyes.
The trick is defending that definition, however. If the definition is assumed, it is quite easy to see why a Catholic isn’t worried about Constantine’s Arians tendency (which would be a point in favor of Nicea not being corrupted by him) or St. Nicholas’s fighting tendencies. However, such a definition is hard to defend in just a post.
Christi pax,
Lucretius
St. John the Baptist, pray for us!