Sri Aurobindo
Letters of Sri Aurobindo
Volume II - Part 3
I do not know that I can answer your question about what Krishnaprem means by Krishna’s light. It is certainly not what people ordinarily mean by knowledge. He may mean the light of the Divine Consciousness, or if you like, the light that is the Divine Consciousness or the light that comes from it or he may mean the luminous being of Krishna in which all things are in their supreme truth,– the truth of Knowledge, the truth of Bhakti, the truth of ecstasy and Ananda, everything is there.
There is also a manifestation of Light – the Upanishads speak of jyotir brahma, the Light that is Brahman. Very often the sadhak feels a flow of Light upon him or around him or a flow of Light invading his centres or even his whole being and body, penetrating and illumining every cell and in that Light 
there grows the spiritual consciousness and one becomes open to all or many of its workings and realisations. Appositely I have a review of a book of Ramdas (of the Vision) before me in which is described such an experience got by the repetition of the Rama mantra, but, if I understand rightly, after a long and rigorous self-discipline. “The mantra having stopped automatically, he beheld a small circular light before his mental vision. This yielded him thrills of delight. This experience having continued for some days, he felt a dazzling light like lightning, flashing before his eyes, which ultimately permeated and absorbed him. Now an inexpressible transport of bliss filled every pore of his physical frame.” It does not always come like that – very often it comes by stages or at long intervals, at first, working on the consciousness till it is ready.
We speak here also of Krishna’s light – Krishna’s light in the mind, Krishna’s light in the vital; but it is a special light – in the mind it brings clarity, freedom from obscurity, mental error and perversion; in the vital it clears out all perilous stuff and where it is there is a pure and divine happiness and gladness.
There are some however who seem to regard this invasion of Light not merely as a thing without value but a thing of evil or, possibly, one that can be such and so to be distrusted: for I have before me a letter describing an experience very similar to Ramdas’s, but it was condemned by the writer’s Guru as an attempt at possession by a devil to be dispelled by uttering the name of Ramakrishna!
But why limit oneself, insist on one thing alone and shut out every other? Whether it be by Bhakti or by Light or by Ananda or by Peace or by any other means whatsoever that one gets the initial realisation of the Divine, to get it is the thing and all means are good that bring it.
If it is Bhakti that one insists on, it is by Bhakti that Bhakti comes and Bhakti in its fullness is nothing but an entire self-giving, as Krishnaprem very rightly indicates. Then all meditation, all tapasya, all means of prayer or mantra must have that as its end and it is when one has progressed sufficiently in that that the Divine Grace descends and the realisation comes 
and develops till it is complete. But the moment of its advent is chosen by the wisdom of the Divine alone and one must have the strength to go on till it arrives; for when all is truly ready it cannot fail to come.