 As the persecution continued, he issued a letter to the bishops of Mexico, "Firmissimam constantiam", and expressed his opposition and granted papal support for "Catholic Action" in Mexico for the third consecutive time with the use of plenary indulgence on March 28, 1937.The 1917 Constitution was drafted by the Constituent Congress convoked by Venustiano Carranza in September 1916, and it was approved on February 5, 1917.The new constitution was based in the 1857 Constitution, which had been instituted by Benito Juárez.