Hi Hamil:
"Salvation" is a bit of a loaded term. It seems to connote "grace", another emotive term. Sadly, a semantic exercise might be needed..."salvation" perhaps from the root "salve", something for relief of a symptom? Or a more religionist, "deliverance from the power and penalty of sin"?
It seems when I hear people speak of the Christain salvation they are bringing in elements of perfect deliverance from a all-powerful source, provided in an unconditional way - yet often predicated on a triggering event from a supplicant (e.g., ascribing to a belief or performing an act, often ritual in nature).
This dynamic seems to be in Christian doctrines, as well as perhaps Judaism and Islam, though in the latter there is perhaps a more marked emphasis on adherence to customary or ritual law as opposed to an internal conviction as the trigger for salvation.
A major difference (to me) between Christian and the more recent Eastern faiths is that Christianity (especially among the fundamentalist and evangelical branches) seems to offer salvation (an internal experience of relief) from the penalty of sin (an internal experience of guilt) in a way that is divorced from the external world. No specific act of falling short or sinning is needed (through the concept of Adamic sin) nor is any external act of contrition necessary, yet the concept of salvation continues to have power and is placed in a primary position. In short, you personally may have never sinned, yet need salvation; you personally may need never act to undo the damage of your sins, yet through the power of belief you can put them behind you. In some belief systems, you may yet continue to sin, yet through the power of your belief (the assertion of your relationship with Christ or the assertion of your belief in him or his power) your salvation is assured (as an internal experience or position).
This seems to differ from Judaism, et al, where salvation is instead achieved as a result of following a "law". The "law" itself doesn't grant you slavation, but following the law will keep you out of trouble or on the good side of the street. This doesn't really seem to be the same as the Christian salvation, because it isn't granted unconditionally.
This contrasts markedly with the Buddhist schools I am familiar with, since Buddhism doesn't really offer salvation. There is no central entity that grants salvation, there is nothing conditional nor unconditional about it. There is deliverance, but this is achieved not through belief nor act: it is just a natural result of living. Suffering in Buddhist thought is a result of attachment (greed or fear), and it is up to the individual to release these to gain peace. While there are practices that lay out ways to achieve this, Christian salvation as an act of grace or the reward of conviction seems different.
That suffering exists (physical, emotional, what have you) does not seem to be solely in the province of religion. The non-religious may have more freedom to determine whether suffering exists, or what causes suffering. So, what the religious would call the need for salvation (suffering) is not necessarily only in religion. Within religion, though, there is often the necessary belief that suffering exists.
Salvation as relief from suffering (alone) has so many possible applications that it cannot possibly be solely a religious expression. It can, like a salve, be a purely medical or physical dynamic.
But salvation from a theofic source is, by definition, something only available through a spiritualistic paradigm - and only through a spectrum of religious conviction.
In that sense, yes, salvation is only available within (and only demanded within) a religious framework.
These are given as theologically based salvation forms:
* being saved from something, such as suffering or the punishment of sin - also called deliverance
Since sin is a religious concept (breaking a divine law), this by definition only takes place within religion. Many religions, though not all, would have this as a hallmark.
* being saved for something, such as an afterlife or participating in the Reign of God - also called redemption
Without semantic appeals, it's hard to see Buddhism proposing a Reign of God - therefore, again, this would not be a "universal hallmark" of religion.
* being saved through a process
If "being saved" might include simple release or relief (without the typical "trappings" of Christian salvation as discussed in the early part of this post), then perhaps most if not all religions offer this kind of "salvation".
I would venture to say that a principal function of religion is to offer an explanation (and a relief) from suffering. A Buddhist perspective might include the notion that "suffering" occurs when attachment causes pain, and while pain is unavoidable you can avoid suffering through enlightenment, so we could include Buddhism in that.
The danger is that "salvation" is a coded word in our culture, and so it merits scrutiny before univeral application.