A Witness (n.) was all Socrates ever needed, and only one: his opponent, whose vote alone he took.
The Witness (n.) who counts is the person you are arguing with: Socrates never called another, but made his opponent's own vote suffice for all.
Witness (n.) for God means testifying that nothing outside the moral purpose is good or evil — and you take the stand to whimper "I am in sore straits, O Lord, and in misfortune."
A Witness (n.) is what God summons you to be: called to testify that nothing outside choice is good or bad, do you take the stand and whine that nobody cares for you?
A Witness (n.) for Zeus is the good man in poverty, sickness and prison — what the lecturers only talk solemnly about as paradox; Zeus exhibits him to show that blessings are found within.