I have been looking at your word "salvation" as the equivalent of our word "theosis". I think I can say with some confidence that when a Roman Catholic uses the word salvation he means theosis but it is evident that you are using it in a different way. I also notice that you use the words "process of sanctification" seemingly as an equivalent with "salvation". For Orthodox Christians and for Roman Catholics, there is a distinction between salvation or theosis and the "process of santification" or the "process of theosis". That process is always as a result, both initially and ultimately, of grace or the uncreated energies of God by the Holy Spirit. +Symeon the New Theologian a man who some say attained theosis in this life, puts it this way:
"Can a man take fire into his bosom, and his clothes not be burned?' (Prov. 6:27) says the wise Solomon. And I say: can he, who has in his heart the Divine fire of the Holy Spirit burning naked, not be set on fire, not shine and glitter and not take on the radiance of the Deity in the degree of his purification and penetration by fire? For penetration by fire follows upon purification of the heart, and again purification of the heart follows upon penetration by fire, that is, inasmuch as the heart is purified, so it receives Divine grace, and again inasmuch as it receives grace, so it is purified. When this is completed (that is, purification of heart and acquisition of grace have attained their fullness and perfection), through grace a man becomes wholly a god."
What he is describing here until the final sentence is the gradual process of theosis. The final sentence describes complete theosis, which the Roman Catholics call 'salvation". Theosis or salvation for us is the "end product" and is not descriptive of the process.
+Thalassios taught the same thing in a different manner; note the final sentence:
"...when the intellect has been perfected, it unites wholly with God and is illumined by divine light, and the most hidden mysteries are revealed to it. Then it truly learns where wisdom and power lie... While it is still fighting against the passions it cannot as yet enjoy these things... But once the battle is over and it is found worthy of spiritual gifts, then it becomes wholly luminous, powerfully energized by grace and rooted in the contemplation of spiritual realities. A person in whom this happens is not attached to the things of this world but has passed from death to life."
Works, as the fruit of the indwelling of the Holy Spirit and also as a preparation of the "self" for more "fire" of the Holy Spirit, play a vital roll in the this process of theosis. As +Gregory Palamas wrote, note again the final sentence:
"After our forefather's transgression in paradise through the tree, we suffered the death of our soul - which is the separation of the soul from God - prior to our bodily death; yet although we cast away our divine likeness, we did not lose our divine image. Thus when the soul renounces its attachment to inferior things and cleaves through love to God and submits itself to Him through acts and modes of virtue, it is illuminated and made beautiful by God and is raised to a higher level, obeying His counsels and exhortations; and by these means it regains the truly eternal life. Through this life it makes the body conjoined to it immortal, so that in due time the body attains the promised resurrection and participates in eternal glory."
Thus for the theology of The Church, what you call "salvation" we call a process, salvation or theosis being what these Fathers are speaking of in the final sentences of the above snips. Now as for falling away during the process, well the Fathers are unanimous in their conviction that indeed that can happen. The process of theosis, theosis itself and the possibility of failure during the process as The Church believes is graphically demonstrated by the icon of the Ladder of Divine Ascent; I don't think I've posted it on this thread:
We start at the bottom of the Ladder and climb upward to union with Christ which is theosis. Along the way we are tempted by demons and encouraged by angels. Some make it, others fall off the Ladder into the Pit (notice that the one headlong into Hell and the third and fourth up the Ladder falling off are hierarchs, very Eastern! :))
Your quotes have not been interpreted by the Fathers or The Church as meaning we cannot fail of our goal of theosis. The first two texts refer, so The Fathers say, to our sealing with Holy Chrism at Chrismation. Indeed, in the Orthodox sacrament, the priest says, as he annoits the Christian, "The servant of God N is sealed in the name of The Father and The Son and The Holy Spirit...." The next verse, from John, is interpreted to mean that while no one can "snatch" us from the hand of God, we can "fall" out of it. As +John Chrysostomos writes in Homily VI on Phillipians:
"As long as we are in the hand of God, “no one is able to pluck us out” (John x. 28.), for that hand is strong; but when we fall away from that hand and that help, then are we lost, then are we exposed, ready to be snatched away, as a “bowing wall, and a tottering fence” (Ps. lxii. 3.); when the wall is weak, it will be easy for all to surmount."
The final quote from Romans is repeated time and again by the Fathers for two purposes. One is to demonstrate how God's love falls on all, the good and the evil equally:
"Love never hates anyone, never reproves anyone, never condemns anyone, never grieves anyone, never abhors anyone, neither faithful nor infidel nor stranger nor sinner nor fornicator, nor anyone impure, but instead it is precisely sinners, and weak and negligent souls that it loves more, and feels pain for them and grieves and laments, and it feels sympathy for the wicked and sinners, more than for the good, imitating Christ Who called sinners, and ate and drank with them. For this reason, showing what real love is, He taught saying, 'Become good and merciful like your Father in Heaven,' and as He rains on bad and good and makes the sun to rise on just and unjust alike, so also is the one who has real love, and has compassion, and prays for all." Abba Ammonas (one of the Desert Fathers)
The second reason is to demonstrate that we all have equal access to God's grace, as +Peter the Damascene writes:
""We all receive God's blessings equally. But some of us, receiving God's fire, that is, His word, become soft like beeswax, while the others like clay become hard as stone. And if we do not want Him, He does not force any of us, but like the sun He sends His rays and illuminates the whole world, and he who wants to see Him, sees Him, whereas the one who does not want to see Him, is not forced by Him. And no one is responsible for this privation of light except the one who does not want to have it. God created the sun and the eye. Man is free to receive the sun's light or not. The same is true here. God sends the light of knowledge like rays to all, but He also gave us faith like an eye. The one who wants to receive knowledge through faith, keeps it by his works, and so God gives him more willingness, knowledge, and power"
So, it seems you are using "salvation" in a different way than we do. Tell me, what do you believe happens after the death of the body? BTW, sorry for being so long winded.