Lord Krishna showed his universal form to his dear devotee friend Arjuna.
No one before Arjuna had ever seen this universal form.
It was because of Arjuna that other devotes of the Lord in the disciple succession were fortunate to see the Lord’s universal form.
Lord Krishna manifested his universal form by his internal potency.
Lord’s internal potency is inconceivable by human brain.
Lord Krishna says to Arjuna that his universal form cannot be seen simply by studying Vedas or by performing pious activities or by sacrifices or by performing penances.
To see the Lord’s universal form one has to be a pure & unalloyed devotee of the Lord.
The supreme Personality of the Lord showed his universal form to Arjuna to clear his doubt.
At the battlefield of Kurushetra Arjuna did not want to fight because the Kauravas were his relatives.
But Lord showed to Arjuna that those people with whom he is going to fight are already killed because of their impious acts.Arjuna was just a doer.
Lord Krishna removed his dear devotee Arjuna’s fear & following the instruction of the Lord Arjuna engaged himself in the warfare.