Salvation is being in the full eternal presence of God in Christ. It is dynamic and timeless union and communion with triune God in Christ.
Nirvana is the natural emptiness of the self, phenomena (including body and mind), and the triune God. Realization of nirvana in buddhahood is complete and permanent realization of all pervasive emptiness.
Hence, buddhahood is the full realization of emptiness (of body-mind-self) pouring into emptiness (of triune God) without division, confusion, separation, or conflation—in a non-dual (not two) and non-monistic (not one) sort of way.
Thus, realization of nirvana is none other than the essence of salvation. This possibility is present in chapter 2 of the book of Romans (see e.g. Romans 2:6-16): perfect good works and perfect keeping of the law can result in salvation.
In this case, perfect development and completion of the eightfold path and the six perfections (including Zen, Highest Yoga Tantra, Mahamudra, and Dzogchen)—the Buddha Dharma—as a type of law and expression of good works can result in salvation of which nirvana is the essence. This process is effected by both the aspirant’s self-efforts and God’s common or prevenient grace.
Jesus as incarnation of God the Son came to offer salvation by grace to all, given that most if not all people are unable by their own efforts to achieve salvation. But sovereign grace provides alternative pathways to salvation through the spiritual genius and rigorous methodology of the Buddha. Thus, a microscopic percentage of humanity might well be able to attain salvation (via nirvana) by sheer self-effort aided by common/prevenient grace. Such grace is commonly interpreted as the blessings and empowerments of the guru-buddhas and bodhisattvas, pacceka buddhas and arahants, patriarchs and ancestors.
Dharma practice pre-regeneration/justification/born-again evinces a self-striving and earning quality. Common grace supports and aids self-effort.
Dharma practice post-regeneration/justification/born-again is refreshingly free from self-striving and earning though suffused with energy and zeal. Sanctifying grace carries and energizes egoless engagement in Dharma.
All my Dharma teachers show profound awareness and insight, spaciousness and egolessness, and a warm compassion that truly cares. This is ample proof of the potency and effectiveness of sincere and authentic Dharma practice. Such wondrous qualities of the Spirit can be summed up in the three doors to liberation—the highest practice of the Dharma across all lineages. These three doors are emptiness (sunyata), signlessness (animitta), and wishlessness (apranihita).
Emptiness, signlessness, and wishlessness can be seen as three aspects of the same realization of nirvana. When one sees the empty nature of all phenomena, penetrating into their absence of inherency, the door of emptiness opens.
When one sees emptiness of all phenomena, one also sees the contingent and nominal nature of the same. While particularities appear, they are no longer grasped as and fixated upon as inherently separate and different entities. The signs that make and mark each entity one apart from another no longer hold the enlightened one in their enchanting spell. This is opening the door of signlessness.
Seeing emptiness and signlessness, one’s heart is freed from afflictive desire and clinging. The wish to acquire, attain, possess, control, manipulate or identify with anything disappears. Preferences may remain but there is no compulsion or obsession in them. Self-referencing and self-occupied desires fall away. This is opening the door of wishlessness.
Empty, signless, and wishless in their inner posture and outer conduct, my teachers evinced the wondrous qualities of enlightenment. These are also demonstrations of the fruit of the Spirit. For by their efforts aided by grace, they realized nirvana—the essence of salvation—and were transformed into the likeness of Christ. How amazing!
I do not know if they encountered Christ during their meditations in life or at the time of death—especially at the transition phase between this life and the next (assuming there is one). It is certainly possible. For Christ can meet us on any road, if He so chooses. I would caution Christian believers, specifically evangelicals and fundamentalists, not to be too hasty in writing off people whom they may regard as being "unsaved." In fact, I am not so sure that all professing Christians are "saved" either, in the absence of any evidence of their "salvation" in terms of the fruit of the Spirit. Humility is always handy.