There are two ways of looking at reality of the Divine – as unmanifest self-existence and as manifested being. As unmanifest, it is the undifferentiated consciousness in which all existences abide and which pervades whole of the universe. It is beyond all dualities of subject and object. It is the divine energy which is nirguna (without any attributes), nirvishesh (without any special distinctions) and nirakaar (without any form). Distinct from the individual soul, the self ; it is the Supersoul which is seated in the hearts of all living beings. Though dwelling in the body, this imperishable Supreme Self neither acts nor gets entangled. It is called Brahman, the Absolute. It is the impersonal God.
On the other hand, as creator, preserver and absorber of the cosmos ; the Divine is called Ishvara, the Supreme Lord. He is a manifested Lord who can be related to as a person, as a being or as an entity. He is not some distant reality but a benevolent deity of love which can be experienced by the devotee. He bestows freedom on those who are devoted to Him. He is the personal God.
God is both – impersonal and personal. From the point of view of the ultimate truth, He is impersonal ; and from a relative viewpoint, He is personal. As impersonal reality, it is an abstract principle that lies beyond the perception of the senses. It is beyond any shape, name or gender etc. And as personal God, He is the Lord who has descended here on earth for establishment of the primacy of righteousness. He is the personification of inexhaustible energy, unbounded love, unlimited compassion and infinite knowledge. Brahman and Ishvara, thus, are not two different realities, but two aspects of the same Supreme Entity.
The Divine in its unmanifest aspect is eternal, beyond the concepts of time and space. It is Pramatman, the Universal Spirit. And as the manifested Lord, He is a separately identifiable entity. He has many forms, features and functions. Though reigning elegantly in His own realm, beyond all dualities and illusions, Ishvara has got all attributes of a living being. He is the god of grace who inspires love, trust and spontaneous self-surrender. He listens to and answers prayers of his devotees. Though as the impersonal concept, it is beyond any emotions ; as personal God, He is an ocean of compassion and mercy.
In the Gita, Krishna is the manifested Lord, the personal God. He is the divine reality in human form. He is the source of all embodied souls. But, He also represents the changeless truth behind all appearances. Krishna has declared that entire creation is pervaded by Him through His unmanifest form (Shloka 9.4).
Bhagavad Gita in shlokas from 12.1 to 12.5 has compared worship of the personal God with contemplation of the Universal Self. Arjuna wanted to know as to who had better knowledge of yoga – those who seek oneness with the Absolute or those who long for unity with the personal form of God. Should one worship Brahman, the Absolute or Ishvara, the Supreme Lord ? In response, Krishna stated that those who fix their minds on Him (the Supreme Lord) ; and worship Him earnestly with supreme faith – they are considered by Him to be the most perfect in yoga.
He further stated that those who worship the formless aspect of the Absolute Truth – the imperishable, the undefinable, the unmanifest, the omnipresent, the unthinkable, the unchanging, the immobile and constant – by restraining the senses, being even-minded in all circumstances, and remaining engaged in the welfare of all creatures, also reach Him (the Supreme Lord). But for those whose minds are attached to the all-pervading formless consciousness, the path of realisation is full of troubles. Search for the unmanifest is more difficult because it is not easy for the human beings to achieve the goal of the Absolute Truth.
To put it in simple words, though worship of the Divine in its manifested form is considered to be the most preferred means of achieving oneness with Him, those who seek Him in its unmanifest aspect also reach the same goal. But the path of search for the formless is more arduous because there is no tangible object to relate to. It is very difficult for anyone to concentrate his mind on the one which cannot be perceived by the senses. On the other hand, while worshipping a deity of his own choice, one has a wonderful enchanting form before him on which he can gladly focus. He can establish a relationship with Him – a relationship of love, reverence and trust. He can beseech Him for anything. He feels relaxed in His presence.
In the ultimate analysis, distinction between the personal and impersonal aspects of God are not of much relevance ; as Krishna (in shloka 4.11) has assured that He accepts men in whatever manner they approach Him, for everyone, in one way or the other, was following His path only.
To conclude, one can worship either of these aspects of the Divine. But no one can claim that only his conception is correct and the view of others is erroneous.