I'm not in the arguing business, but I have noticed that virtually everything that we do that you guys also do, we have an older version of. Even the so-called "controversial" stuff (which is really just controversial for your church, not ours) like the "addition" to the Trisagion or the way we make the Sign of the Cross, is all explained with reference to earlier sources that show that our way of doing things usually preserves what was done in the East before the establishment of distinct to the Eastern Orthodox Church. Note, for instance, that even
Eastern Orthodox web resources like Orthodoxwiki say that the use of more than one finger to make the sign of the Cross is a later development from what was likely the original, one-finger way (which is how we still do it in the OO church). Similar, too, the "addition" to the Trisagion which the EO think is heresy because they do not respect or understand our traditions, nor usually seek to understand them (thank you for being an exception) is reported by writers of antiquity -- OO and EO alike -- to predate the founding of the Eastern Roman Empire itself, so there can be no doubt that this is the original way
with reference to Antioch from which it spread. From the OO, we have sources like Zacharias of Mitylene (5th/6th century bishop, historian, and contemporary and biographer of St. Severus of Antioch) who traces the origin of the "thou who wast crucified for us" phrase in the Trisagion to the time of Patriarch Eustathius, who reigned from 325 to 330 AD (recall, the Byzantine Empire itself was only founded c. 330 AD). From the EO, we have people like Chalcedonian Patriarch Ephrem of Amida (d. 545), who recorded of his travels in Antioch that this is was the common tradition of the people of the region, without noting any difference between his Chalcedonian party and the non-Chalcedonians.
These are but two simple examples. And of course I am sure our EO friends would have their own historical sources that they would point to vis-a-vis the particular instances mentioned above, which is fine. What is important is not who crossed themselves which way, or which version of history about the Trisagion is the one that 'really' happened (as if that is even possible to know when both EO and OO say that the prayer came to them by miracles! I refuse to have a 'miracle duel' with anyone), but how we live. I can point to the 6th century inscription found in the Monastery Church of St. Mark, the center of the Syriac Orthodox Church in Jerusalem, that says
"This is the house of Mary, mother of John, called Mark. Proclaimed a church by the holy apostles under the name of the Virgin Mary, mother of God, after the ascension of our Lord Jesus Christ into heaven. Renewed after the destruction of Jerusalem by Titus in the year A.D. 73", but I can't make you or anyone believe that what it says is true. And I can show you videos of HE Mor Severus Malki Mourad, the Syriac Orthodox archbishop of Jerusalem, praying the holy qurbono of the Syriac Orthodox Church in the tomb of St. Mary in the closest thing to the language of our Lord that anyone still speaks, but that's not going to have any effect on you if you already think of the Eastern Orthodox as the original, true, first church. You'll say "but the gospels were composed in Greek!" "the liturgy was originally served in Greek at X location!", "X location was a Greek-speaking city at the time of Christ!" And you won't be wrong about any of that. It's just a different way of looking at history, and since neither of us will adopt the other's way, it's silly to fight about it. But if we live in a Christ-like manner, guided by holy fathers who teach us humility and compassion, we will be too busy in prayer and fasting for arguments. And this is best, I think. Even better than being right on the internet.
And yet, since you asked, I do have to say that these are some of the reasons that I think we are the original church. In a more general sense, it is important to note that we never went through a distinct period of "Byzantinization" as the eastern Chalcedonian churches did, so even if you do not believe that the OO are the earlier of the churches, you can still look to us as evidence of how your own church probably worshiped prior to the Byzantinization of the Chalcedonians in the Near East. We know, for instance, that the Syriac language still survived among the Melkites in the Levant and Egypt until maybe a few hundred years ago (I've read everything from the 13th to the 17th centuries, and I don't know who is to be believed about this), and there is plenty of evidence that the Copts and the Melkites probably shared many more liturgical similarities originally than they do now. You can see that by looking up the development of the different forms and usages of the Liturgy of St. Basil among the Christians in Egypt, or for that matter the Basilian canons which govern the Coptic Orthodox Church -- the vast majority of these are particular to the Copts (i.e., have no EO equivalent), but there are also 13 shared with the Melkites (EO), pointing to a common origin in the writings of St. Basil himself, who of course lived before the Chalcedonian schism. Even after the Chalcedonian schism, there is evidence of closeness between us, such as when your Patriarch Timothy III (Salophakiolos, d. 481) got in trouble with the then still-living Latin Pope Leo I for his continued mention of HH St. Dioscoros, the Coptic Patriarch of Egypt, in the diptych of
Chalcedonian church of Alexandria. Whoops.
So even if we disagree, it is not all bad.