No human or a person can ever claim that he / she has complete knowledge or truth, since no human can ever gain complete knowledge or know exact truth. Only Allah or God or Yahweh or Bhagwan or whatever other name you have for a Supreme Being can ever claim or say that because only It knows the what the real truth is.
Allah revealed that truth in Quran & whoever reads it, without any biases or presumptions, & with an open mind, starts to see the real truth & gain that knowledge, which will benefit him / her the most in this mortal life & in the immortal life in the hereafter.
Disclaimer: Yes, Albert Einstein was an atheist, & hence, most likely didn't believe in the latter half of the quote.
