Arjun has doubt to who he should worship either Sakaar or Nirakaar.How we could imagine that God has a form.Even touht the Arjuna clearly knew that God Almighty was Nirakaar in form... still, engrossed in emotional thoughts when faced with relatives and friends in the opposite army... his faith in God Almighty dwindled in this confusion.
When Arjuna sought guidance from Lord Krishna to understand difference between worshipping God in Sakaar form (having a form) or Nirakaar form of God (having no form)... Lord Krishna said: O Arjuna, one who while concentrating ones mind on me indulges in Bhajan Kirtans and worships me in Sakaar form (having a form)... such seekers of God is a Yogi most acceptable to me.
Lord Krishna further says to Arjuna that total submission to God Almighty in such cases were essential, necessitated. Unless one had absolute faith in God Almighty (termed Shraddha in Hinduism) one could never come near to God. Bhajans Kirtans carried no meaning if one indulged in them merely as rituals.
Total submission to God Almighty demanded indulgence in karma that was all the time directed towards God Almighty. Furthermore, one should always be free of all attachments, bondages of material world. One should always see God in all living beings. Only then a human being was competent to travel spiritual path... indulge in bhakti yoga, karma yoga as was meant to be.
Verse 3 & 4
Lord Krishna while expounding the difference between worshipping God in Sakaar form (having a form) and Nirakaar form of God (having no form) tells Arjuna that human beings who after establishing absolute control over five senses finally succeeded in controlling power of mind. They ultimately realize that God can only be reached via Nirakaar form of God (having no form).
Truly God Almighty always was Nirakaar in form (having no form). God Almighty (termed Brahman in Hinduism) is that entity which from beginning of time was embodiment of truth but only truth, beyond perception of senses and mind, was eternal, unchanging... existing from beginning of time, could neither birth or die. To reach such an unmanifest one required yoga of highest order. Yogis for whom entire humanity becomes one large family, unable to distinguish one from another... one who all the time engaged in welfare of entire mankind finally succeeded in reaching God forever.
Believers of the fact that God Almighty was one and only one (termed Brahman in Hinduism)... such Yogis par excellence, true seekers of spirituality... believing in impartiality of God Almighty finally paved their way to abode of God (kingdom of God)... termed Vaikuntha in Hinduism! Only a true believer of God finally realizes that God was everywhere and beyond comprehension of most human beings. With limited power of senses, mind and intellect (buddhi)... one could never comprehend in totality what God Almighty was all about.
Those who believed in the fact that God Almighty was the only truth of life and manifest world was just an illusion... finally reached God in their lifetime! Bhagavad Gita chapter 12 verses 3 - 4 finally confirms that Nirakaar form of God (having no form) was the only and right way to worship God. Worshipping God in any other mode other than Nirakaar form of God (having no form) was meaningless on path of pure spirituality