7 They are created now, and not long ago; you have not heard of them before today. So you cannot say, ‘Yes, I knew of them.’

Read Isaiah 48:7 Using Other Translations

They are created now, and not from the beginning; even before the day when thou heardest them not; lest thou shouldest say, Behold, I knew them.
They are created now, not long ago; before today you have never heard of them, lest you should say, 'Behold, I knew them.'
They are brand new, not things from the past. So you cannot say, ‘We knew that all the time!’