Answer by Niraj Bidawatka
The most important instruction of the Bhagavad Gita is that Krishna is the Supreme Personality of Godhead (Bhagavan) and that we should surrender unto Him alone. We should understand that all the other so-called gods are in fact demigods, who are appointed by Krishna to manage this material world and therefore we should not divert our devotional service to such lesser demigods. Therefore Krishna, in His final instruction at verse no. 66 of Chapter 18, specifically states that one should surrender unto Him ALONE. No one else.
At the present moment we have the tendency to surrender to some powerful person or deity, whom we may consider to be potent enough to supply our needs and satisfy all our desires. Therefore Krishna says “Give up all so-called religions and just surrender unto Me alone. I will take care of you. Don’t worry” So this is the most important instruction of Gita.