It concerns the rite of baptism. He wanted "Church of God" to be said instead of "Christian community." The order to change was issued a few days before his resignation. And it went into effect after the election of Francis
From the article:
In practice pope Joseph Ratzinger, as a sophisticated theologian, wanted that in the baptismal rite it should be clearly said that it is the Church of God - which subsists fully in the Catholic Church - that receives those who are being baptized, and not generically the “Christian community,” a term that also signifies the individual local communities or non-Catholic confessions, like the Protestants.