What is Necessary for Salvation?

In the Orthodox Tradition our salvation is not a simple matter. It is not a matter of being chosen or simply the work of Divine Grace through faith. Our salvation is a long process that is part of a spiritual maturing with the full cooperation of both God and man. We must cooperate with God based on faith, receiving His grace to do righteous deeds.

Here is how Elder Cleopa puts it:
Our Church, however, teaches that our personal salvation is neither a gift, nor a simple work, but rather a process and an undertaking that matures or develops gradually and is realized in the co-operation of two persons: God and man. On the part of God, Divine Grace (His uncreated Divine Energy) is offered to us, while for man's part, faith and righteous deeds are necessary. Consequentually, the prerequisites for our personal salvation are the following: the Divine Grace or uncreated Divine Energy of God and the faith and virtuous deeds of man.... Holy Scripture is filled with passages which refer to good deeds as a necessary prerequisite for our salvation.
Central to this issue is the question of free-will. Does man have free-will? On this question Holy Scripture is very clear. Here are some passages identified by Elder Cleopa:
O Lord as with a shield of Thy good pleasure hast Thou crowned us. (Ps 5:13)
He Himself made man from the beginning, and left him in the hand of His council. (Eclus 15:14)
He hath set fire and water before thee: stretch forth thy hand unto which ever thou wilt. (Eclus 15:16)
Behold, I set before you this day a blessing and a curse; a blessing if ye obey the commandments of the LORD your God. (Dt 11:26-27)
See, I have set befor thee this day life and death, good and evil... I call heaven and earth to record this day against you, that I have set before you life and death, blessing and cursing; therefor choose life, that both thou and thy seed may live. (Dt 30:15,19)
Jesus said to the rich man, If you would enter into Life, keep the commandments. (Mat 19:17)
God says through mouth of Isaiah: If ye be willing and obedient, ye shall eat the good of the land. But if ye refuse and rebel, ye shall be devoured with the sword. (Is 1:19-20)
If you will be perfect go and sell all that you have and give it to the poor, and you will have treasure in heaven; and come follow me. (Mat 19:21)

All the above show that it is obvious that God gave man free-will. Therefore, we must use it to cooperate with God's grace which we receive though faith.
Remember what Paul wrote: For we all must appeared before the judgment seat of Christ; that everyone may receive the things done in his body, according to that he hath done, whether good or bad. (Cor 5:10)
we must remember to be humble when reading Scripture. The True Wisdom of God cannot be gained through intellectual means. We first must purify our minds and hearts from all passions.
Elder Cleopa says,
Christ has brought salvation to everyone, something the theologians have labeled general salvation. And yet, everyone does not actualize this objective salvation, only those who seek and peruse it... Those who desire to be saved and work toward this goal receive divine Grace as their aid and guide. This Grace does not work in us violently; rather it abides with us perennially as a specific offering for the work of our salvation....
Grace does not compel anyone. Men have the God-given freedom to accept it and to work with it or to reject it. Those who embrace it are saved and those who withdraw from it are lost.

Reference: The Truth of Our Faith, pp 153-166