I agree, but then how do we answer those who observe that there were churches all over the world in these centuries, and if they were not denominations, then why were they not denominations, and what is it that makes a local Church a denomination or not?
There always has been non-denominational congregations.
The church is US - the whole corpus of those with the divine gift of faith in Christ as Savior, the whole corpus of those incorporated into His Body, His Oikos, His Church, the church that is one, holy, catholic, communion of believers... one faith, one Lord, one baptism. PEOPLE.... Christian people.... together.
It should be appreicated that SOME Christians are not a part of ANY parish/congregation.... they are non-churched, independent of any parish. They are still Christians and still equally a part of His Church but not a part of any parish. BUT understandably (and IMO by divine command) these Christians gather.... assemble.... congregate.... (the meaning of the word "church") in a given locale and time. In Protestantism, we technically call these "congregations" (from the word to congregate or gather). They are not THE church (although we - like Scripture - use that word in this different sense) the the believers in it are the Church (or part of it - although the church cannot be divided), joined of course with unbelievers (seekers and even hypocrites). They gather for mutual love, support, growth, ministry, accountability - and for Word and Sacrament. Some of these are non-institutional, just an extremely informal affair of people gathering in a living room... some are highly institutional with legal articles of incorporation, constitution and bylaws, officers, property, budgets, etc. There are several of these mentioned in the Bible itself..... one in Corinth, one is Ephasus, one in Thessalonica... seven mentioned in Revelation. A few of these were founded by Apostles but probably most not (certainly not the one in Rome). We must appreciate that for 300 years, the church was ILLEGAL, often persecuted and thus at times "underground" and more a movement than anything.... so the institutional elements were rare in this time (the last thing you want is a membership list or an address the persecutors can use!). While a level of mutual love and care existed among some of these congregations. I note the Council of Jerusalem and the collection for the brothers and sisters in Jerusalem. But there is no denomination in these 300 years....
Denominations are formal, institutional "assemblies" of congregations. Less formal is when each congregation is self-contained but is part of a greater whole and submits to such on some level to some degree.... more formal is when the denomination actually owns and operates the congregations. Denominations are congregations that are legally, economically, geopolitically joined in order to give each other mutual support, cooperation, ministry, accountability. My parish's denomination began our parish (loaning it a LOT of money and giving us a founding missionary for a couple of years) and still supervises our pastor (whom it trained).., we give a min. of 10% of our total income to the denomination. It has lots of programs to help and support the member congregations, it operates the largest Christian publishing house in the world (Concordia), two seminaries, 12 colleges, and it sends missionaries to some 50 countries... agencies of it do still more, LHM for example still has the most popular Christian radio ministry in the world. When a congregation has an "issue," there are people and agencies in the denomination there to help. Member congregations help and and support and cooperate with each other. And yes, we are all pledged to teach in accord with the Lutheran Confessions. There are thousands)of denomiantions today (over 300 of them are Lutheran), but there is no evidence of any until the 4th Century, when the Roman Empire created one for itself. Some aspects of denominations precede that but no actual denominations.
Of course, there have always been congregations that simple are not a part of any formal denomination. They are, by definition, non-denominational ("non-denoms"). They are independent. Even in the early days, there were isolated congregations from China to Ethiopa to Ireland - never a part of the Roman Church. Even the Roman Church itself soon split (creating a loose federation of Oriental Orthodox). Although rather rare for centuries, they are common again. Today, southern California is filled with them.... indeed, most of the largest churches in California are proudly "non-denom." By brother is a member of one of these. "No Creed but the Bible! No denomination divisions! Just Christians!" is their motto. OCCASIONALLY, non-denoms are created because some denomination kicks them out.... and they may "find" each other and form a new denomination, but generally non-denoms stay that way, appreciating their independance. Non-denoms are just as much a congregation as any other - just independent, non-joined with others.
Where there Christians before the Roman Empire created the Roman Church? Yes. Were there Christian congregations prior to that? Yes, perhaps thousands of them. Are there denominations today? Yup, thousands. Are there congregations that are non-denom and not a formal part of any denomination? Yes, millions of them. Where there Christians in the British Isles before Rome founded the Roman Church? Yup. Where there congregations before that? Yup. Were those congretions owned and operated by the Roman Church or the Russian Orthodox Church or The Lutheran Church - Missouri Synod? Nope, none of those denominations existed yet - especially not in the Brtish Isles.
I hope that helps.