During the Mahabharata war Lord Krishna bestows a special “vision” to Arjuna so that he can see the Divine God in His universal form. In this Cosmic or Universal Form, Lord Krishna shows His opulence's, hundreds of thousands of varied divine and multicolored forms. Lord Krishna shows Arjuna, different manifestations of Adityas, Vasus, Rudras, Asvini-kumaras and all the other demigods. Lord Krishna also shows in His Universal Form many wonderful things which no one has ever seen or heard of before.
Vishvarupa has innumerable forms, eyes, faces, mouths and arms. All creatures of the universe are part of him. He is the infinite universe, without a beginning or an end. He contains peaceful as well as wrathful forms. Unable to bear the scale of the sight and gripped with fear, Arjuna requests Krishna to return to his four-armed Vishnu form, which he can bear to see. Fully encouraged by the teachings and darshan of Krishna in his full form, Arjuna continued the Mahabharata War.
In Chapter - 10 of Bhagwat Gita, Arjuna enquires from Lord Krishna about the various forms through which Lord is pervading in all these worlds in His divine opulence. To this query of Arjuna Lord Krishna replies that, "Of lights He is the radiant sun; of immovable things, He is the Himalayas; among the stars He is the moon; of purifiers He is the wind; of the wielders of weapons He is Lord Rama etc. etc.
Now as per Chapter - 11 of Bhagwat Gita, Arjuna asks Lord Krishna to show him the Cosmic Form of God, the way Lord Krishna just described Himself. So Lord Krishna showed Arjuna the Universal or Cosmic form. Arjuna saw in that Universal Form, unlimited mouths, unlimited eyes, unlimited wonderful visions. The form was decorated with many celestial ornaments and bore many divine upraised weapons. He wore celestial garlands and garments, and many divine scents were smeared over His body. All was wondrous, brilliant, unlimited, all-expanding.
"ihaika-stham jagat krtsnam
mama dehe gudakesha
On seeing such a fierce form of God, Arjuna gets frightened & asks God, “Who are you in this fearful form & what is Your mission in this form?”
To this query of Arjuna "Lord Sri Krishna says: Time I am (Kala), the great destroyer of the worlds, and I have come here to destroy all people. With the exception of you (the Pandavas), all the soldiers here on both sides will be slain."
"If hundreds of thousands of suns were to rise at once into the sky, their radiance might resemble the effulgence of the Supreme Person in that Universal Form."
Arjuna now realizing the might of the Lord apologizes to Lord Krishna, that he has considered Him as his friend & rashly addressed the Lord, “O Krishna,” “O Yadava,” “O my friend,” not knowing His divine glories.
Arjuna asks Lord Sri Krishna to return to his “intimate, beautiful form (saumya-rupa)." At Arjuna's request, Lord Krishna, The Supreme Personality of Godhead, again displays His real four-armed form and at last shows His two-armed form, thus encouraging the fearful Arjuna.
The above said article explains well that we should not talk lightly of God & should show full reverence to God.