"Less overtly, [AJC European director, Zachariah] Shuster found other ways to obtain restricted information, and even copies of secret documents. He developed a clandestine source of information, a "mole" within Cardinal Bea's Secretariat. The secret agent was an Irish Jesuit, Malachi Martin, a voluble, larger-than-life figure variously referred to as "Forest," "Pushkin," and Heschel's "young friend" in Schuster's confidential reports and transcripts of transatlantic phone conversations. Martin ... was sympathetic to the Jewish position ...
"With a mixture of motives, lofty and ignoble, Martin became close to Heschel and Schuster. He enjoyed their company immensely, especially when they vied with each other in telling jokes in Yiddish ... Martin primarily advised the AJC on theological issues, but he also provided logistical intelligence and copies of restricted documents." (Spiritual Radical: Abraham Joshua Heschel in America, Edward K. Kaplan, Yale University Press, p.243)
Cited sources for this information are an interview with Malachi Martin, a letter from Martin to the author, and a file on Zachariah Schuster at AJC/NY.
Documentation of AJC payments to Malachi Martin for his services and more here:
On Rabbi Abraham Heschel who Martin as "Heschel's young friend" refers to: