Part 1: Immaculate Conception
At the moment that Mary agreed and accepted God's plan to conceive His Son with her, her "original sin" was cleansed retroactively back to the moment of her own conception. God is eternal and omnipotent, so this is not a problem for Him, even though we can't wrap our finite human minds around it. Our temporal sense, demanding a one way flow of time, balks at the thought of God's mastery of eternity. But this is not an impediment for God.
Therefore, before Mary accepted her vision to become Theotokos, she possessed the same original sin as we have, but she was living an exemplary life. She was already a saint, and so, chosen by God, who knew her from eternity. In the very moment that she said, " Behold the handmaid of the Lord. Let it be done to me according to thy Word," she was free. By putting her will in accord with God, she showed her faith, and God accepted that as righteousness. God cleansed her, going back to the moment of her conception to remove her original sin.
It was just like when we accept Jesus and receive baptism. God removes our own sin, from the moment that we arise from the water. Always, it requires our choice and discipline to align our will with Him, if we want to become saints. We can only do it because God chooses us, but we cooperate by faith and trust in Him. God had chosen Mary, because she is/was/would be worthy to be a pure vessel, and He knew her from the beginning.
In a divinely real sense, Mary was "conceived without sin" because God worked outside of time, as Eternal Being, to make it so, when she showed that she was worthy. She had to be, since God cannot mingle with sin, and He had to cleanse her for the Holy Spirit to come and rest upon her. Because she was totally dedicated in love and service to God, she was full of grace, choosing a saintly life. God is both eternal and omnipotent, and can do whatever He wants, so time becomes a fluid, mixed up factor that we do not comprehend. Mary was a totally, fully and completely normal human woman person, both before and after her vision and her conception of Jesus. And by her participation in God's plan, we are all saved.
Part 2: Against Heresy
If you say that the Roman Catholic Church is teaching heresy and false doctrine, you clearly don't know what you are talking about. The Holy Spirit abides with us to teach us all that we need to know about Christ and his loving work in Heaven, in our souls and in history. Indeed, it is the Orthodox Church that has fallen away from the Truth, since the time that they refused to accept the "filioque" being added to the creed defining the faith. It should have been obvious that the Holy Spirit proceeds from the Father "and the Son." The word was imperative for the defense of the faith against the Arian heresy. Necessity required it, regardless of whether any single apostolic bishop had enough authority, and the Holy Spirit thus granted us a clear understanding. The Orthodox Church simply refuses to speak the whole Truth. That refusal, for merely political principle, was stubborn and ill considered, and should not impede others. Any person of the Holy Trinity always works in concert and full consensus with the other two persons, because together they are only one entity, One God. As Jesus said, "I and the Father are One."
Moreover, if you don't accept a need for the "substitutionary atonement" of Jesus' sacrifice on the cross then you are not saved. Why else would He have to die, and why, before, would God have tested Abraham to find out if he would be willing to sacrifice his son for God? He was asking if a human could love Him enough to be worthy of such a sacrifice. But a merely human sacrifice would not do, any more than animal sacrifices could suffice. It had to be his own, because it was his own decree that demanded judgment and justice.
And yet there are some teachings of the Church that stem from considerations of financial interests, convenient interpretations or lust for power. They were means by which the Church, through its priestly hierarchy, intended to lead the faithful, but they became inflated and calcified by centuries of usage and Papal abuse of authority. The current practices and doctrines do not extend all the way back to the Apostles, but developed after the time of the Early Fathers. This does not mean that they are necessarily false, but that they need not be universal. Other modes of guidance into right behavior and proper piety can also be countenanced. I am referring to issues that were highlighted by the Protestant Reformation, such that some degree of laxity could be approved for the sake of rapprochement. (It might be noted that these are the same three temptations that Jesus faced, and that some Protestant church leaders have been not quite immune to them either.)
Part 3: Christ is Our Only King
The Catholic Church claims to be the One True Church, with ultimate authority over all Christians, on the basis of one single Scripture. When Jesus asked his disciples, "Who do you say that I am?" Simon bar Jonah answered quickly, Thou art the Christ, the Son of the Living God." Jesus approved of this answer, and said, "thou art Peter, and upon this rock I shall build my Church," and gave the keys of the Kingdom. (Matthew 16) And ever since Jesus' Ascension into Heaven, the Apostles have passed on their authority by ordaining leaders of the various regional Churches.
The Catholic Church claims that their bishops can trace their lineage all the way back to Peter, who was the first leader, or "Bishop," of Rome. And this unbroken chain of authority, they say, links them to Jesus' designating Peter as his choice to lead the Church. We can, by humble generosity, grant that this chain is probably unbroken, and thus the Bishop of Rome does deserve a significant level of respect. So, if Peter was the leader of the Apostles, then Rome is the leader of the Church. Right?
And this sounds like a perfectly logical line of reasoning, but it forgets one tiny thing: 1,000 years of church history. The first leader of the Church, beginning in Jerusalem, was James, the brother of Jesus, not Peter, and the first Gospel Churches spread from there. For three whole centuries, Christians had to endure bitter persecutions and struggle against virulent heresies to maintain its identity, and they did this by mutual support and consensus on the doctrines of faith. Leadership by bishops was one of these consensual developments.
Up until the Great Schism, in 1054 AD, the Church recognized five centers of leading Bishop's authority: Jerusalem, Antioch, Constantinople, Alexandria and Rome, and they never conceded ultimate authority, or even perennial leadership to the Bishop of Rome. That is why we had ecumenical councils to determine the correct faith. If Rome had had such authority as they now claim, they could have simply asked the Pope. That never happened.
It took many centuries, until long after the Apostles, and the Patriarchs and then after them, the Church Fathers, including many of the Doctors of the Church, for the "Holy Roman Catholic Church" to presume to be the default leader of the western Church. This was long after barbarians repeatedly sacked Rome, Muslim pirates disrupted trade by sea and the normal modes of communication were broken. Then, centuries later, the Great Schism came as a result of a political power dispute, over a few minor issues that could have been ironed out given time. After that, the traditions and practices began to diverge. But the claim of Papal "infallibility" came only a couple of centuries ago. So, it's farfetched to make the claim that Jesus gave total authority to Peter. Even the doctrine of ecclesiastical traditions can't support a claim that the whole Church must bow to Rome.
The Church was built on "the Rock" of faith. We must believe that Jesus is the only begotten Son of the Living God, and then, that we all as Christians have the authority to forgive each other's sins, just as we pray in the "Our Father," "... forgive us our sins as we forgive those that sin against us." Every integral Church has some community structure for this, perhaps simply to "speak to one another in love," or to informally "confess one to another," which can help its members to remain in a state of grace. And of course, we have ALWAYS been able to take our honest confessions straight to Jesus Himself, and get his perfect absolution and, if He asks, do the penance He suggests. If you lack full faith in your own prayers, and want to hear it from "the Church," you could go to a priest for confession, but Jesus is our High Priest, not the Pope. The ultimate authority belongs to Christ the King.
Part Four: Reparations
When the Pope wants to reunite the Church, he will have to take the first step by inviting the other branches to come and negotiate. The Catholic Church represents the majority opinion, but we have to be considerate of all concerned. We cannot stand on our own arrogance. In order to begin, he must agree to recognize the dignity of the other original "Sees" of orthodoxy, and take a step back from his audacious claims of authority. He needs to be willing to listen to the opinions of the other Patriarchs, and submit to the consensus, as before the schism.
And as Bishop of Rome, he will have to grant forgiveness, and formally remove the anathemas and excommunications for the Reformation, and embrace any and all who would rejoin the worldwide Body of Christ, on the only condition that they too submit to the consensus, in which case, their voices would also be heard. That would reunite the whole Church. Some steps have been made in this direction, but many more remain. It may be that another worldwide ecumenical council will need to be convened, even more broadly inclusive than Vatican II, with promises not to overwhelm the other Churches, Orthodox or Protestant, by sheer numbers, and to be respectful of their traditions and practices.
And of course, we should be glad to be ever reminded, we have much more in common than we have of differences. There was a thousand years of unity before the Schism, and we still say the same Nicene Creed. If we still want to retain the unique flavors of our different cultures, there should be a fair and amicable way to do that, and to extend the embrace of brotherhood across the seas that now divide us, in recognition that the whole world is now one, and they are now all "Maria Internus."
But one caveat remains. There is no peace in this world. We still await Our Lord's return. The Church, and thus all Christians as the Body of Christ, is still suffering persecution from political empires that refuse to allow and recognize the spiritual authority of our God, even when He orders us to obey earthly civil authorities, and seek to gently guide them to righteousness. And we still face the threat of heresy from the apostate cult of Islam, which seeks to overthrow Christianity and conquer the world for its own power and glory. Both of these threats suffice to remind us that we may yet be faced with the dreadful choice: to deny our faith, or to suffer imprisonment, torture or death. For this reason alone, if for nothing else, we need to stand together in solidarity, relying on and sharing Jesus' love with one another. So, for now, "May peace be with you."