They are created now, and not in that time. And even before today, you did not hear of them; otherwise, you might say, "Behold, I knew them.
Other versions
King James Version
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.English Standard Version
They are created now, not long ago; before today you have never heard of them, lest you should say, 'Behold, I knew them.'New International Version
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.'