The concept of God, karma, heaven and hell are ideas created by religion, to help for those who are incapable of grasping the ultimate truth of Brahman.

People are not aware of the fact that,  there is no God can exist, apart from  formless soul or consciousness, the innermost self  . If there is no  soul, the innermost self  , then there is no body, world and their belief of god. They think that there must be a creator of this universe. If one thinks self is body , then there is a creator, but if one thinks the soul is  Self, then there is nothing exists other then the soul.

If one objectifies and sees a universe, then he is bound to see many things beside himself and postulate a God, the creator. Body, God and world rise and set together from, and into, the soul. If God is apart from the soul or consciousness, the innermost self , then He would be Self-less, that is, outside existence, that is, non-existent.

In Isa Upanishad says:

MANTRA 10

Scholars say that the path of Avidya [performing Aganihotra and other sacrifices] and the path of vidya [worshipping gods and goddesses] produce different results. Wise men confirm this. [X]

Vidya and Avidya both are hindrances to Self-knowledge, but Vidya is even worse than Avidya. The word Vidya is used here in a special sense; here it means worshipping gods and goddesses. By worshipping gods and goddesses you will go after death to the world of gods and goddesses. But will that help you? The time you spend there is wasted, because if you were not there you could have spent that time moving forward towards Self-knowledge, which is your goal. In the world of gods and goddesses you cannot do that, and thus you go deeper and deeper into darkness.

Avidya is Karama and therefore a hindrance. You perform Avidya - i.e., you perform Agnihotra and other sacrifices. This is a roundabout way of purifying the mind, and it is also groping in the dark. But it may not have as heavy a toll on your time and energy as the other.

The final advice by sankara is: combine both work and worship. Together they may quicken your pace towards Self-knowledge, because it will lead to Chitt shuddhi, purification of the mind. When this happens your desire for enjoyments will become less and your sense of "I' and "mine' will subside. This is the way to Karama-Mukthi, gradual, or progressive, liberation, according to Sankara.

MANTRA 11

He who worships gods and goddesses [Vidya] and also performs sacrifices [Avidya] attains immortality by sacrifices [Avidya] and attains bliss by worshiping gods and goddesses [Vidya]. [XI] 

The condition in both cases worshipping gods and goddesses and performing sacrifices is that the person should have no motive for personal gain. He should not desire the fruits of his actions, such as going to the heaven of gods and goddesses. 

As earlier explained, the word Vidya ¸ has a special meaning here. It means worship of gods and goddesses. Similarly, Avidya ¸ also has a special meaning. It is Karma that is, performing Aganihotra and other sacrifices. Such karamas are obligatory, but if they are done without any attachment to their fruits, they help purify the mind. Combining karama and worship is a path to gradual liberation. Sankara approves this for those who are not yet ready to renounce. 

But suppose you follow the two paths separately. If you perform Avidya ¸ you will go to Pitra loka (the world of your ancestors). This is a dark region, because it is far removed from Self-knowledge. In fact, you will have to wait a long time to attain Self-knowledge. But if you worship Vidya ¸ - that is, gods and goddesses you go into still darker regions, and your attainment of Self-knowledge is even further delayed. 

True, you go to Dev loka (the heaven of gods and goddesses), but you are caught in the pleasures there, and you remain there until the fruits of your worship are fully exhausted. You are then reborn as a human being, and your struggle resumes from where you left off. This is why Vidya ¸ is considered worse. But if you combine the two that is, perform the obligatory karmas without any attachment to the fruits and at the same time worship gods and goddesses, again without any desire to go to heaven - then you can get the benefit of both, liberation and bliss. For those who are not yet ready to renounce, this path is recommended.

It proves that,  the concept of God, karma, heaven and hell are ideas created by religion, to help for those who are incapable of grasping the ultimate truth of Brahman.. The concept of god and goddesses and karma are part of the ignorance. By worshiping god and goddesses one accepts the false self as true self, thus he is unfit for Self-Knowledge or Brahma Gnana  or Atma Gnana . Therefore concept of god is not the means to acquire the self knowledge. That is why Buddha rejected concept of God.