I often hear it stated here that devas only have causal bodies and subtle bodies, and do not have a gross body. Where does this idea originate? Which scripture or scholar says this?
Subtle Panchabhootas are imperceivable by earthly species.Hierarchy of Atma ,Akasha,vayu ,Agni ,Aaapa,Prithivi is applicable only for sukshma (subtle) Panchabhootas. (Panchatanmatras)
Sthula(gross) panchaboothas obtained by combining sukshma panchaboothas( explained in Panchikaranam, quintuiplication) can only be perceived.
One cannot see a sukshma panchaboothas even by augumenting their sense organs with a device. So Devas subtle corporeal form is absolutely inaccessible through sense organ.
Indeed , Gross corporeal forms aren't completely in the locus of perception, either...A plant ot a worm can never ever comprehend human form etc, notwithstanding, those jivas are also the stakeholder of the entire ecosystem.,
Anyway,the notion of sthula and sukshma is not general across earthly species. Whatever is sthula for a dog (say supersonic frequencies) is sukshma for a human , and viceversa .
So, to truly know who devas are , One has has to do the tapas / sadhana .
Rishi Badarayana points out in Brahmasutra (devatadhikaranam) , that devas are also corporeal beings ,(doesn't mean they are visible to humans) entitled for the experience of Brahma Jnana.Anectodes of Devas like Varuna and Indra, achieving Jnana is quoted in the upanishads as well(ch up 8.7.11).Also, a particular Deva can be simultaneously invoked in different sacrifices , held at different places, (ref. BrahmaSutra).Their simultaneous apperance is also cited in Brahma sutras.
Nirukta , says (Dyusthano bhavathi) one who lives in celestial region , Amarakosha would describe devas with words as bright, splendid etc.
The devas, or celestials. While in this world they remain dissolved in the five elements, in the celestial world they exist in a visible form. Those who have obtained siddhi or perfection by chanting the mantras can also see them in their gross form in their celestial abode besides receiving their blessings in their subtle form. The gods emanated from the Paramatman as a result of the vibrations produced by the mantras. We may therefore describe the mantras as the "sonic" form of the deities The deity appears during a sacrifice when he is invoked with mantras. Those who are wise and mature will perceive them with their eyes. Even if they do not, the power of the deities will be subtly revealed to them. However, offerings cannot be made directly to them. When you write a letter you have to stick a stamp on it or put the seal of the registrar. According to the "regulations" of the Vedas, any oblation intended for the celestials must be offered in the sacred fire in a form acceptable to them.
Sage of Kanchi , Pujyasri Chandrashekhara Saraswati Swami , Hindu dharma the universal way of Life 5.19