I think the first thing to note is that Bulgakov is an Orthodox theologian. Which means that, for Bulgakov, salvation is more ontological than forensic.
By forensic I mean the Protestant focus on human guilt and divine forgiveness. For many Protestants, these forensic ideas are captured by what is called penal substitutionary atonement. In this view, being "lost" means standing under the judgment of God as a sinner. Salvation is having this judgment removed by Christ who paid/atoned for our crimes by taking the penalty upon himself.
For the Orthodox, by contrast, being "lost" isn't a forensic issue but is, rather, an ontological predicament. Due to sin, human being is weakened and made vulnerable to the powers of sin and death. It is this weakness and vulnerability that demands attention. Consequently, where Protestants have exclusively focused upon the death of Jesus in their forensic discussions of salvation, the Orthodox have focused upon the Incarnation, the Resurrection, and Pentecost. For in these events God comes to strengthen human being, and through this connection triumph over the powers of sin and death. Thus, the Orthodox vision of salvation is less about "forgiveness," a forensic focus, than upon theosis, the divinization of human being. Salvation is an ontological transformation.
With that understanding in hand, you can anticipate how Bulgakov would place Sophia at the heart of salvation. Again, Bulgakov's issue is divine mediation, how God relates to creation. In a conversation about salvation, then, especially one with an ontological focus as with the Orthodox, the question of grace concerns how God works to rehabilitate human being that has been wounded by sin. How does the divinization of human being and creation happen? As Bulgakov says, what we need here is an "ontology of grace." He writes:
Two aspects are naturally distinguished in the doctrine of grace: the sending down of grace by God's power and its reception by creation. The relation between these two aspects and their effects, as well as the foundation and ultimate goal of this relation, must become the object of theological (or, more precisely, a sophiological) interpretation.
Crudely speaking, how do nature and grace "dock" when they "make contact," what is the nature of this relation?
For Bulgakov, the "docking" between nature and grace is sophiological. Grace comes to us from the Divine Sophia (the divine life of God) which draws the creaturely Sophia (the created world) to herself. The path of salvation is sophiological. The creaturely world soul is slowly drawn into a participation with the life of God. Western theologians describe salvation in a similar way, as coming to participate in the life of the Trinity. The Orthodox sophiological tradition agrees, but digs deeper, trying to explain the "how" this could happen, an attempt to understand the "ontology of grace." For example, what do you mean by "the life" of the Trinity? What is this "life" you speak of? Well, for the Orthodox sophiological tradition the life, or ousia, of God, is Sophia. Thus, to be drawn into the life of God is to be drawn into the Divine Sophia. And how can this happen? Because, as we've seen already in this series, human being is already grounded in the divine life, already, as the creaturely Sophia, connected to the Divine Sophia. And because of this ontological connection, divine connected to divine, creaturely Sophia to Divine Sophia, there is, if I can but it this way, a natural path for theosis. The sophiological connection between God and humanity is that path. As Bulgakov says, "[The] ontological possibly of 'salvation' through deification is predetermined by the very creation of man in the image of God."
Simply stated, if salvation is an ontological predicament, a question of how human being is to be reconnected with and drawn into the life of God, the Orthodox sophiological tradition answers that question by positing a divine connection between God and humanity. That is to say, since human being is grounded in divinity it is predisposed to enjoy divine life with God. In the Incarnation, Resurrection and Pentecost, a sophiological pathway is established between God and humanity. The road is opened for theosis.
There are three critical points at work in this vision.
The first is that the wound of sin never wholly or completely severs our sophiological connection to God. Again, because we exist, we are rooted in divinity. And that divinity continues to maintain a bridge to God. As Bulgakov says, "This connection [between the Divine and creaturely Sophia] is never interrupted, never terminated, for otherwise the foundations of being, unshakably laid by God, the Creator and Almighty, would crumble away." Sin does not erase the image of God in humanity. As Bulgakov argues, the effect of sin is "only a certain darkening...of the image of God with the weakening of freedom, though man is seen as preserving the capacity for good in counteraction to evil. Therefore, man's creative power is, in general, preserved." Because our humanity is founded upon divinity, our fall cannot be a total catastrophe. Sin darkens the image of God within us, and weakens us, but the image is not eradicated.
Secondly, sin does not destroy human freedom. Nor does God override human freedom unilaterally. God works with human freedom relationally. God cooperates with human freedom. Bulgakov's vision is synergistic, God's will and our will is a partnership. As Bulgakov argues,
Divinity can act upon the person only by interacting with it on the basis of creaturely freedom...[God] acts without coercing; that is, He persuades, limiting his power to the measure of creaturely receptivity. This is precisely synergism, as the form of divine providence with regard to human beings...
Divine providence is therefore a dialogue of God's wisdom and omnipotence with free creaturely life...Providence shows absolute skill and inventiveness in correcting and fulfilling the actions of creaturely freedom [in] guiding the world to salvation; but it always respects the originality and freedom of creation...The relation of the Creator to creation in "synergism" always remains meek and restrained, the kenosis of God in creation...God waits for creaturely freedom...
Ontologically, man cannot get rid of freedom even if he so desires, for it is the mode of the very being of the creaturely spirit...Creaturely freedom is naturally afflicted by selfhood, from which it can free itself only by voluntary self-renunciation, in the death on the cross. Therefore, the supreme freedom is the one that is manifested in the obedience of the Handmaid of the Lord: "be it unto me according to thy word" (Luke 1:38)...Through this acceptance we discover this will that acts in the whole life of the world. This faith in divine providence does not paralyze our creative activity in freedom and does not condemn us to the quietistic passivity of fatalism. On the contrary, it reinforces our will to search for the right way to accomplish God's will in us and through us, in true "synergism."
An obvious point to make about all this is that Bulgakov is completely rejecting Calvinism, its view of "total depravity," the bondage of the will, and the monergistic doctrine of election, where God alone acts in effecting salvation.
The final point to make concerns how grace relates to human nature. As noted above, given the sophiological connection between God (Divine Sophia) and the world (creaturely Sophia), human nature has an innate, natural capacity for God. As Bulgakov argues,
Between "natural" or creaturely being and supernatural grace there must be an ontologically positive relation, without which grace would be deux ex machina, an ontological violence done to creation, and not the elevation of creation to its proto-image [in the Divine Sophia]...
The existence of "natural" grace is a necessary precondition for the reception of grace in the strict sense. This reception presupposes in creation a conformity with divinity that is actualized in deification by grace. "Natural grace" is precisely the humanity that contains the image and likeness of God. In virtue of the divine image and likeness, human beings are called to Divine-humanity, which is the union of the two natures in Christ. Divine-humanity extends to all humankind, which possesses "natural grace" or sophianicity by its creation...
All the aspects of grace and the modes of its bestowal have as their sole purpose and content the elevation of creatures to deification, the imprinting of the image of divinity in the creaturely likeness. Outside of this relation, in the absence of "natural grace," that is, in the absence of the conformity of creation with its Creator, such an imprinting would be an ontological coercion, just as impossible as the transformation of an ape into an angel or a stone into a human being...
The vision here is that God uses this innate and natural connection to grace to cooperate, synegistically and relationally, with the human person to draw us deeper and deeper into the divine life. Grounded in divinity, humans have a natural capacity for grace. Our nature is to seek and rest in God. Sin doesn't erase this capacity. Sin does, however, weaken us, which means that God must actively seek us. Otherwise, we'd be lost. But we, for our part, must also reach out and seek God. Salvation engages us in active participation. As Bulgakov summarizes:
Grace is not a divine coercion over the human nature; it is not something accidental, something alien to man, that does not have to be, that exists as a kind of happy caprice. On the contrary, grace is the actualization of the Divine-humanity of which the seed was implanted in man by virtue of his creation in the image of God and which was accomplished through his communion with God in the Incarnation and the Pentecost. Grace gains this power in the gradual and unceasing approach towards one another of the divine nature and the human nature. In grace, man knows and realizes the foundation of his proper being. "Not I, but Christ liveth in me" (Gal. 2:20), by the Holy Spirit, who unites God with man. Life in Christ is given to man, and is also proposed to him as a goal; he is a temple of the Holy Spirit.