When the tzaddik prays on behalf of a sick person, and the like, his prayer is answered. Apparently, that indicates that the simple, true Creator is changing, heaven forbid.
But the root idea is that Hashem created the letters, which are in potential with Him. The tzaddik combines these letters so that they will be read in accordance with his will—i.e., the tzaddik prays by means of combining the letters. And so there is no change in God, heaven forbid. The letters existed from the beginning, only that the tzaddik combines them.
It is specifically the tzaddik who is answered in his prayer. As our Sages said, “A person who has a sick person in his house should go to the tzaddik and pray” (Bava Batra 116a). But why is that? Why shouldn’t every person pray and the letters will be combined?
The answer is that the holy Torah was created with love. As the siddur states, “He Who chooses His nation of Israel with love.” The tzaddik also loves Hashem and every person in the world. As Rabbi Yochanan said, “No one ever greeted me first on the street, not even a gentile” (Berachot 17a). But that is not the trait of every person. Therefore, the letters only combine for the tzaddik who loves all.
And thus “Yaakov sent angelic messengers,” these being letters and words. He sent them “to Esav his brother,” meaning that even Esav was complete with him as his brother.
Noam Elimelech: Parashat Vayishlach