Non-Buddhist couldn't attain Jhana, because real jhana requires suppresion of 5 hindrances. Without belief in triple gem you cannot suppress the 5th hindrance. Without understanding of anatta, you cannot fully suppress the the first two hindrances.
Some might object saying Alara Kalama and Udakka Ramaputta has attained aruppas, and by inference Jhanas. However, It is not very certain that they have achieved the real Jhana states. More likely those teachers have achieved a diluted version of those states. If they did, why didn't it lead to Nibbana, while Jhana is the path to awakening (MN36, MN52)? If they taught "Jhana", then why did the Buddha remembered his Jhana as a child and not their Jhana states? Why was it only the Buddha that has awoken to Jhana (SN 2.7) and discovered the N8P that leads to them?
What about AN9.36 that says:
"'I tell you, the ending of the mental fermentations depends on the first jhana.' ... "'I tell you, the ending of the mental fermentations depends on the the
dimension of nothingness"
http://www.accesstoinsight.org/tipitaka ... .than.html
If Alara Kalama really attained base of nothingness, then why didn't he become Awakened?
In Dhp 372 there is a statement that "there is no Jhana without panna".
So how could non-Buddhist ascetics have Buddhist panna? The middle path was discovered by the Buddha not them. The other two common paths was either self-mortification or indulgence in sense pleasures. None of these are particularly suitable for Jhana.
"Suppose that a wild deer is living in wilderness glen. Carefree it walks, carefree it stands, carefree it sits, carefree it lies down. Why is that?
Because it has gone beyond the hunter's range.  In the same way, a monk — quite withdrawn from sensual pleasures, withdrawn from unskillful qualities —
enters & remains in the first jhana:.... This monk is said to have blinded Mara. Trackless, he has destroyed Mara's vision and has become invisible to the Evil One." http://www.accesstoinsight.org/tipitaka ... .than.html
If Jhana blinds "Mara", then it has to have panna. Furthermore if Brahmas are Jhana attainers (and are in a jhanic states) then why can they be possessed and
controlled by Mara (see MN49 sutta for example).
Buddha: "You are Mara, Evil One. And Brahma, and Brahma's assembly, and the attendants of Brahma's assembly have all fallen into your hands. They have all fallen into your power. http://www.accesstoinsight.org/tipitaka ... .than.html
Again if Brahma and his attendants are said to posses Jhana, then how can they fall under Mara's hand if Jhana blinds Mara?
Note: It is not necessary according to the suttas to develop Jhana to be reborn in rupa/aruppa loka. See mn120.
Again the bhikkhu is endowed with faith, virtues, learning, benevolence and wisdom. The bhikkhu learns, that gods born in the sphere of consciousness ... re ... in the sphere of nothingness, re ... in the sphere of neither perception nor non perception have a long life span and enjoy much pleasantness. It occurs to him. O! I should be born with gods of the sphere of neither perception nor non perception with long life, much pleasantness. This is the path and method to be born there.http://metta.lk/tipitaka/2Sutta-Pitaka/2Majjhima-Nikaya/Majjhima3/120-sankharuppatti-e.html
In MN127 it says that one can have impure, with hindrances of sloth&torpor metta meditation and yet be reborn in rupa loka. Without overcoming ALL hindrances (including sloth & torpor, doubt, etc), one isn't technically even in full Buddhist First Jhana.
http://metta.lk/tipitaka/2Sutta-Pitaka/ ... dha-e.html
Some quick quotes
Jhana IS the path to awakening - MN36
Jhana Is what Buddha awakened to. AN9.42 & SN2.7
Jhana Is Practiced by awakened ones: Dhp 23
Jhana Leads to 4 fruits: From Stream to Arhatship. (DN29)
Jhana Is Right Concentration - SN 45.8
Ending of Mental Fermentations depend on Jhana - AN 9.36
Samadhi is proximate condition to "knowledge and vision of things as
they really are" - SN12.23
Samadhi -> seeing rise & fall of 5 aggregates (which is wisdom) SN 22.5
Samadhi -> wisdom
Samadhi is the path - AN 6.64
Jhana is the only 4 Meditative absorptions thay Buddha praised. -MN108
Jhana goes together with discernment (panna): Dhp 372
Jhana Is a mark of a great discernment, great man - AN4.35
Jhana is the escape from confinement. AN 9.42
Released through Panna (Pannavimutti) = Jhanas 1-9 AN 9.44
7 Parts of Noble 8Fold path are support for Jhana- MN117
Jhana + discernment is a single thing that can lead one to Arhatship - AN 11.17
The Buddha has recomended Jhana for trainees - MN107
It is *impossible* to break 5 lower and 5 upper fetters without Jhana (and
insight after it). - MN64 Jhana are 4 stations of mindfulness (anussatithana)
AN 3.322 Udayin sutta iii, 320, VI, iii, 29