"He then moves on to describe the origins of Atlantis. He said that Atlantis was allotted to Poseidon. Poseidon fell in love with a mortal girl named Cleito (daughter of Evenor and Leucippe), and she bore him a number of children, the first of which was named Atlas, who inherited the kingdom and passed it onto his firstborn for many generations. Critias then goes into a great deal of detail in describing the island of Atlantis and the Temple to Poseidon and Cleito on the island, and refers to the legendary metal orichalcum. Critias then reiterates the remarkable virtue of the Atlanteans, saying:

"For many generations, as long as the divine nature lasted in them, they were obedient to the laws, and well-affectioned towards the god, whose seed they were; for they possessed true and in every way great spirits, uniting gentleness with wisdom in the various chances of life, and in their intercourse with one another. They despised everything but virtue, caring little for their present state of life, and thinking lightly of the possession of gold and other property, which seemed only a burden to them; neither were they intoxicated by luxury; nor did wealth deprive them of their self-control; but they were sober, and saw clearly that all these goods are increased by virtue and friendship with one another, whereas by too great regard and respect for them, they are lost and friendship with them."
However, the Atlanteans became corrupt as:

"...when the divine portion began to fade away, and became diluted too often and too much with the mortal admixture, and the human nature got the upper hand, they then, being unable to bear their fortune, behaved unseemly, and to him who had an eye to see grew visibly debased, for they were losing the fairest of their precious gifts; but to those who had no eye to see the true happiness, they appeared glorious and blessed at the very time when they were full of avarice and unrighteous power."
Critias then says that Zeus, the god of gods, seeing the corruption of the Atlanteans, determined to chastise them. Zeus begins to speak; but what he says, and everything that follows in the Critias, remains non extant."
en.wikipedia.org/wiki/Critias_(dialogue)