The Orthodox Church is supposed to be “the” Church.
Yet, the Catholic Church is also supposed to be “the” Church.
Was the ancient Catholic Church Orthodox before the Great Schism. . ?
I do not deny that Luke65 is right.
I must add that if so, somewhere sometimes the unique position of Papacy was neglected.
On the other hand, if orthos are right, the horrid filioque heresy was well and thriving among people whom they themselves also count as saints (Pope Leo was just mentioned by Luke65, his letter to the bishop of Astorga indicates he believed the Holy Ghost was eternally proceeding from BOTH the Father AND the Son: St Augustine of Hippo believed that too - see De Trinitate, as did St Hilary of Poitiers, and if Quicumque Vult is genuinely his - orthos tend to deny it - St Athanasius of Alexandria too).
Dioscuros condemned by abovementioned sentence has also followers to this day, known as Copts. They and Armenians tend to say that Catholics and Orthodox are Christians too and overdid condemnations without falling into heresy back in Chalcedon. They do however condemn Nestorians - who also exist to this day.
So, looking for the primitive Church is not like looking for a needle in a hay-stack, you can pick Catholics, Orthodox, Copts-and-Armenians, or Nestorians.
And they do have some substance in common against protestantism, like Seven Sacraments, like Priests offering a true Sacrifice in Holy Mass, like priesthood depending on a special class of priests called Bishops, like recommending Celibacy, especially to priests and Bishops, like having Celibate Monks and Nuns who say the psalms of David on certain hours each day, some disciplines all 150 in a day (Copts, I think, and Coptic Rite Catholics), some in a week (Catholics and Orthodox), some in two weeks (Novus Ordo Catholics). Or like admitting some men as Canonised individual examples of what Sainthood is. We know very well that Saints means all Christians. We also know very well that some Christians are not fully Christians as in being real Saints. We then propose certain Christians as already known examples, guaranteed b y Divine Miracles. And that means we do NOT admit the age of miracles ended with the last apostles. We do all accept the Blessed Virgin Mary as greater than all other saints, including the angels whose nature is more noble than the human one.