Advaita is Gyana Yoga and ISKON preaches bhakti yoga. The word “Bhakta” is the opposite of the word “Abhakta” which means without any splits, as in “the whole” which is Gyana Yoga. Thus Bhakta literally means Split in sanskrit.
These are two entirely different techniques meant for 2 different sets of people. Both Bhakti yoga is correct and Gyana yoga is correct as it’s true that there is a separation between your real pure self and your present state. That separation is of the veil of ignorance or Maya. It is this ignorance that doesn’t let you see the reality. And the root of this ignorance is Ahankara or the “I”, the ego. Thus the entire effort of a bhakta is to lose this I and yearn to become one with the divinity. The eventually the oneness occurs. And that is the highest truth of advaita vedanta, non-duality.
Osho says in the Bhakti marga you lose the ego in the first step and accept divinity as the only reality.
In Advaita or Gyana marga, you lose the ego in the last step. Thus Shanakaracharya sings “ I am not the senses, not the thoughts, not the ego…. “ Here we see eventuali the “I “ remains. And in the final step the individual “I” becomes the “I” of that one non- dual. “ Aham Brahmasmi” claims the enlightened. But that realisation is dangerous if people start contemplating on it. This is the highest realization, not a mantra.
We know the story of Mansoor in Islam. He declared “ An-al-Haq” which meant :I am God. He was stoned to death for blasphemy. Islam follows the path of Bhakti and thus saw Mansoor as Kafir. But Mansoor just expressed his realisation.
Coming back we see, in both Bhakti and Advaita, only the Super “I”, the non dual “I” remains. The seeker’s I is dissolved in that super I. But the paths are strikingly different. Thus the teachings are entirely contrasting too, and hence the animosity. An Arya samaji would not like the ISKON people and vice versa. If only they understood the one truth.