But how does one "fall out of Christ?" Based on what I'm reading here and elsewhere it seems pathetically easy. I might even be tempted to call it "saved one day, damned the next."
Not at all. The only "mortal sin" according to the Orthodox tradition is refusal to repent (more on that below).
How can God "take over the controls" when you're supposed to be working with Him?
"'For it is God which worketh in you both to will and to do according to good will.' (Phil. 2:13) What could well be clearer than the assertion that both our good will and the completion of our work are fully wrought in us by the Lord? And again 'For it is granted to you for Christ's sake, not only to believe in Him but also to suffer for Him.' (Phil. 1:29) Here also he declares that the beginning of our conversion and faith, and the endurance of suffering is a gift to us from the Lord." - St. John Cassian, The Conference of Paphnutius, 15
This returns to my former question, how many of us will repent at all times? "Be perfect as your Father in Heaven is perfect." Isn't that what it takes to be saved? How do you repent and obey 51% (or even 99%) of the time?
God saves repentant sinners. He does not parse us mathematically, to ask "how much is repentant and how much is sinner?" It is not the one who fails a mathematical equation that will be lost, but the soul who refuses to repent is in danger of being lost. Not "fails to repent X amount of the time," but refuses to repent.
Heb 12:25: "See to it that you do not refuse him who speaks. If they did not escape when they refused him who warned them on earth, how much less will we, if we turn away from him who warns us from heaven?"
We are made perfect on a relational basis
as we abide in the mercy of the Perfector, partake of His flesh and blood, find mercy through the prayers of intercession which are offered continually, and the intercessions offered by the Holy Spirit when we do not know how to pray which are beyond expression in human words:
Rom 8:26: "In the same way, the Spirit helps us in our weakness. We do not know what we ought to pray for, but the Spirit himself intercedes for us with groans that words cannot express."
Christ Himself continues to intercede for us also, as we are told in Hebrews. This should make it plain that our forgiveness is not a "done deal" finished in a single Grand Moment of uttering the Sinners Prayer; if all the future sins were expiated a single instant in the past, what on earth would there be left to intercede for?
ORTHODOX PRAYER OF REPENTANCE
"O Lord our God, good and merciful, I acknowledge all my sins which I have committed every day of my life in thought, word and deed; in body and soul alike. I am heartily sorry that I have ever offended thee, and I sincerely repent; with tears I humbly pray the O Lord; of thy mercy forgive me of all my past transgressions and absolve me from them. I firmly resolve, with the help of Thy Grace, to amend my way of life and to sin no more; that I may walk in the way of the righteous and offer praise and glory to the Name of the Father, Son, and Holy Spirit. Amen."
We need absolution "for all things wherein in word, or deed, or thought, and with all... senses, whether voluntary or involuntary; whether through knowledge or ignorance... may all those things which have proceeded from the weakness of mortal nature be consigned to oblivion, and be remitted..." (The Prayer of Absolution).
Now you might say, how can we admit we sin every day of our lives and resolve to sin no more? Jaroslav Pelikan has remarked that most major heresies in Christian history emphasize one pole of a dialectical dogma. We are not big enough to look at the call to repent and "pursue ...holiness without which no man will see the Lord" (Heb 12:14) and the reality that there is no man who lives continually and never sins, and conclude we must only preach one side and pitch the other: EITHER human moral perfection as St. Cyril rebuked the heretic Novatus for doing, OR scrap the notion that we should take seriously the call to pursue holiness, or repent continually (repent in the NT is frequently in the Gk. continual present -not a Grand Moment, but a lifestyle of repentance is in view there), die to the self, etc. Like the mystery of the incarnation we Orthodox say yes to all of it -no half Gospel or truncated Gospel for us!- and we pray to live it until our last breath.
"Even if we have thousands of acts of great virtue to our credit, our confidence in being heard must be based on God's mercy and His love for men. Even if we stand at the very summit of virtue, it is by mercy that we shall be saved." —St. John Chrysostom
God saves repentant sinners "in Him" who abide in the Vine. That relational basis is dialectical: we come to Christ praying "Lord have mercy!" and "Jesus Christ, Son of God, have mercy on me, a sinner!" We are not lost because we cannot repent enough seconds of the day, but if we refuse to repent, and thus unto the ages of ages (cf. Heb 10:26ff.).
"What answer then will those make to this, who embrace the new tenets of Novatus, and say of themselves that they are pure? Whose prayer do they praise? That of the Pharisee, who acquitted himself, or that of the Publican, who accused himself? If they say that of the Pharisee, they resist the divine sentence; for he was condemned as being boastful: but if that of the Publican, why do they refuse to acknowledge their own impurity? Certainly God justifies those who know well their transgressions, and are willing to confess them: but these men will have the portion of the Pharisee. We then say, that in many things we "all of us offend," and that no man is pure from uncleanness, even though his life upon earth be but one day. Let us ask then of God mercy; which if we do, Christ will justify us: by Whom and with Whom, to God the Father, be praise and dominion, with the Holy Spirit, for ever and ever, Amen." -Cyril of Alexandria, Homilies on the Gospel of Luke, Sermon 120
"If we say we have no sin, we are deceiving ourselves and the truth is not in us." -1 John 1:8
“Imitate the Publican and you will not be condemned with the Pharisee. Choose the meekness of Moses and you will find your heart which is a rock changed into a spring of water." -Sayings of the Desert Fathers
"...anybody who thinks he is something great, even before God, is rightly abandoned by God, as one who thinks that he does not need His help." -St. Gregory Palamas, Discourse on the Publican and the Pharisee
"When the foolish thought of counting up any of your good works enters into your head, immediately correct your fault and rather count up your sins, your continual and innumerable offenses against the All-Merciful and Righteous Master, and you will find that their number is as the sand of the sea, whilst your virtues in comparison with them are as nothing." - St. John of Kronstadt
"The Pharisee went up to the temple with a proud and empty heart; the Publican bowed himself in repentance. They both stood before you, O Master: the one, through boasting, lost his reward, But the other, with tears and sighs, won your blessing: Strengthen me, O Christ our God, as I weep in Your presence, since You are the lover of mankind!" -Lenten Triodion