Uri Tzvi Greenberg was one of the foremost Hebrew and Yiddish poets of the twentieth century. Born into a chassidic family in Galicia, he immigrated to Eretz Yisrael in 1924. His poetry, steeped in Jewish sources and in the national experience, including the Holocaust, earned him the Israel Prize. He served in the first Knesset for the Herut party.
Several of these letters concern the "Who is a Jew" question in Israeli law; they present the Rebbe's own position.
"Who is a Jew." Much of the correspondence, from the late 1960s, concerns the "Who is a Jew" (Mihu Yehudi) issue. The Rebbe urged unwavering public advocacy for defining Jewish identity according to halachah in Israeli law (IK 9889), warned against complacency and silence (IK 9943), and specifically urged Greenberg not to remain silent but to use his unique talents publicly in this cause (IK 9979).
Faith and talent. The Rebbe wrote to him that faith precedes doubt and critique, being innate in every Jew (IK 9723), about the power of Tanya study (IK 9741), and encouraged him to use his unique gifts to inspire others toward G-dliness, assuring him that obstacles are there to be overcome (IK 10338).
Sources: Wikipedia ("Uri Zvi Greenberg"); and the Rebbe's letters cited above.