In Night what was Rabbi Eliahou's effect on prisoners?

This is a
good question. Rabbi Eliahu is only mentioned twice in the book. So, he did not make an impact
on the prisoners. We can assume that since he was a religious figure, some prisoner's found
comfort in his presence. Others probably did not care one way or another. However, what we can
say is that the Rabbi made an impression on Elie for a specific reason. 

The
first time the rabbi is mentioned, the rabbi is talking with Elie. He lost his son, and so he
was asking Elie if he by chance saw him. Elie said, "no." 

Later
Elie remembered something. He realized that he did see the rabbi's son. The son ran ahead and
even peeked back to see his father. He continued to run, because he did not want to be held back
by his aging and weakening father. The situation was so bad that sons betrayed fathers.
Self-presevation ruled. 

When Elie realized this, he prayed that he would
never become like the rabbi's son. 

"Oh God, Master
of the Universe, give me the strength never to do what Rabbi Eliahu's son has
done."

When the rabbi is mentioned again, Elie
begrudgingly took care of his father. When he realized this, he knew that he experienced the
heart of the rabbi's son. 

I gave him what was left of my
soup. But my heart was heavy. I was aware that I was doing it grudgingly.

Just like
Rabbi Eliahu's son, I had not passed the test.