The Hebrew of Genesis 15:13 is severe enough: know with certainty that your seed will be a stranger in a land not theirs, and they will afflict them four hundred years. Targum Pseudo-Jonathan adds a sentence that many readers flinch at, and should.
It reads: thy sons shall dwell in a land not their own, because thou hast not believed.
The Targum is making a bold theological move. The exile, it says, has roots not only in the future but in a moment of Abraham's own hesitation — a flicker of doubt the rabbis located in his earlier question, how will I know that I shall inherit it? (Genesis 15:8). That tiny hesitation, says the paraphrast, echoes forward for four hundred years of slavery.
This is hard, and the Maggid does not soften it. The Targum is not blaming Abraham for Egypt. It is refusing to let even the greatest patriarch off the hook for the long reach of a single doubtful word. Faith matters. Faith's absence matters. And sometimes the bill comes due in the lives of grandchildren (Genesis 15:13).
But there is mercy inside the severity. If the exile has a cause, the exile has an end. Four hundred years is a long sentence, but it is a finite sentence. A door opens at the far side. Abraham sees all of it in one horrified heartbeat — his slip, his children's suffering, and the morning beyond it. The Targum teaches that no exile in Israel's history is random. Every night has a reason, and every reason has a morning.