I think while the Orthodox and Lutheran perspectives on a surface level can look very similar they are at their core very distinct and very much opposed to each other.
Justification by faith alone may be the big thing preventing the two churches from ever uniting. In that, its not that Orthodoxy believes we somehow work to earn our salvation, as if by doing actions we can pay with them to God to enter into paradise, but we stress the need the for change, that we ought not be content in remaining sinners but instead perfect ourselves and obey the commands of Christ. That we should “seek righteousness,” Not be like the unprofitable servant, “abstain from every kind of sexual immorality” and “Be perfect, therefore, as your heavenly Father is perfect.” Yet at the same time the ORthodox prayer books say things like this, “o lord heavenly king, comforter and spirit of truth, show compassion and have mercy on me your sinful servant and remit and loose me from my unworthiness, and forgive all wherein I have sinned against you today as a man, not only as a man but even worse than a beast…”
There is a quote from Saint Mark the ascetic and his “On those who think they are made righteous by works,” which I think encapsulates the Orthodox position.
“Wishing to show that to fulfil every commandment is a duty, whereas sonship is a
gift given to men through His own Blood, the Lord said: ‘When you have done all
that is commanded you, say: ‘We are useless servants: we have only done what
was our duty’ (Lk 17.10). Thus the kingdom of heaven is not a reward for works,
but a gift of grace prepared by the Master for his faithful servants.”
But in the same breath he also says:
“He who relies on theoretical knowledge alone is not yet a faithful servant: a faithful
servant is one who expresses his faith in Christ through obedience to His
commandments” and further on “He who honours the Lord does what the Lord bids. When he sins or is disobedient, he patiently accepts what comes as something he deserves.”
From what I see in the Orthodox patristic tradition we embrace an emphasis on both faith and works whereas in Lutheranism the sole focus is on faith alone, our complete and utter unworthiness before Christ, that he is the sole reason for which we are saved and we do nothing to effect that. There is little emphasis in Lutheranism (from what I have seen) like there is in the orthodox church on the need to do good works. Perhaps the Lutherans may charge me with not detailing their position but from what I’ve read and heard from Lutherans is the emphasis on faith alone. I am not saying that Lutherans deny works, I know they believe works are the natural outcome of faith. Perhaps the main distinction is in the monergism and synergism dichotomy. The Orthodox believe man can work with God, that we can be receptive to him and embrace him and then with God’s help or guiding hand as it were we are made Holy and “saved.” That is we can of our souls accept the help that God offers. God reaches out, we accept it and we are saved and continue on to glorification in the hopes of resurrection in glory. I will leave it to a Lutheran to explain monergism which I believe centers everything in God, that we are not doing anything ourselves but it is solely God through us who does whatever is good.
There are also other practical matters which differentiate us. The Orthodox locates the church with those group of churches which bear apostolic succession in both faith and the Bishops who are the successors of the apostles. Lutherans feel no need to have Bishops in a line of succession from the apostles, believing their faith is enough to justify the ordination of Priests, Bishops and deacons.
There is also sola scriptura. In the end the Lutheran judges everything by what he sees in scripture. This is demonstrated by the charges of the Lutherans made to the Patriarch in the correspondence made by others that on certain doctrines and practices that they found it not in scripture and thus they were not persuaded. Some of the practices the Lutherans of that time rejected are the following:
Monasticism (they didn’t explicitly reject it but they found it wanting in justification from scripture): Second Exchange of Tubungin to the Patriarch “We know Basil became a great admirer of the monastic life. But there are times when even the most renowned men, in judging, overlook some things (which you yourself have said before hand…) so that, consequently, nothing on earth is found absolutely perfect and blessed in everything. Therefore, we demand divine and not human evidence without purpose.”
Veneration of icons: Moreover, the sixth and seventh ecumenical synods are not of such great esteem as to have authority in the church of God to introduce new forms of worship without the sanction of the divine word. Neither can it be denied that many of the canons which have been decreed in the synods are not entirely applicable to the rule of faith of the divine word. Therefore, since neither the prophets, nor Christ, nor the apostles ordered the invocation of the saints or the reverencing or worshiping of their icons, no one perhaps is able on this account to condemn us for impiety because we only omit that which God himself in his own words did not require of us, second exchange. 31 (Modern Lutherans have told me that they personally accept the seventh ecumenical council and find nothing wrong in what it says)