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Śrī Īśopaniṣad
devotee. Lord Nṛsiṁha appeared from within the pillar not at the order of the atheist king, but by the wish of the devotee Prahlāda. An atheist cannot order the Lord to appear, but to show mercy to His devotee the Lord can appear anywhere and everywhere.
The  Bhagavad-gītā  confirms this, in the statement that the Lord appears to vanquish the unbelievers and to protect the believers. To vanquish an unbeliever, of course, the Lord has sufficient energies and agents who can do the work, but to favor the devotee is a pleasing task for Him, and therefore He descends as an incarnation. He does so only to favor His devotees, then, and not for any other purpose.
In the  Brahma-saṁhitā  it is said that Govinda, the Primeval Lord, enters everything by His plenary portion. He enters the universe as well as the atom of the universe. He is outside in the  Virāṭa  Form, and He is within as  Antaryāmi . As  Antaryāmi  He is witness of everything that we are doing, and He gives us the result of our actions as  karma-phala . We ourselves may forget what we have done in previous lives, but because the Lord is the witness of our actions, the result of our actions is always there, and we have to undergo the reactions all the same.
In fact there is nothing except God within and without. Everything is manifested of His different energies, like the heat and light of fire, which means there is a oneness among the diverse energies. In spite of this oneness, however, the Lord enjoys in His Personal Form all that is enjoyable to the little senses of the little part and parcel living entities.
MANTRA SIX
यस्तु सर्वाणि भूतान्यात्मन्येवानुपश्यति । 
    सर्वभूतेषु चात्मानं ततो न विजुगुप्सते ॥ ६ ॥