। ॥ १८ ॥
In plain words
When ‘Brahman is Ātma — my own Self’ becomes as firm a conviction as ‘this body is Ātma — myself,’ everything that had to be done is done. Let this body die whenever it may; such a one is free. Of this there is no doubt.
Close to the Sanskrit
When the conviction that Brahman is Ātma — the Self — becomes as firm as the conviction that the body is Ātma — oneself — there is the state of having accomplished what had to be done. Whenever this one may die, that person is liberated; there is no doubt here.