ŚB 1.13.49

There are two classes of human beings, namely the envious and the obedient. Since the Supreme Lord is one and the father of all living beings, the envious living beings are also His sons, but they are known as the asuras. But the living beings who are obedient to the supreme father are called devatās, or demigods, because they are not contaminated by the material conception of life. Not only are the asuras envious of the Lord in even denying the existence of the Lord, but they are also envious of all other living beings. The predominance of asuras in the world is occasionally rectified by the Lord when He eliminates them from the world and establishes a rule of devatās like the Pāṇḍavas. His designation as kāla in disguise is significant. He is not at all dangerous, but He is the transcendental form of eternity, knowledge and bliss. For the devotees His factual form is disclosed, and for the nondevotees He appears like kāla-rūpa, which is causal form. This causal form of the Lord is not at all pleasing to the asuras, and therefore they think of the Lord as formless in order to feel secure that they will not be vanquished by the Lord.