The New Leaven
"The kingdom of heaven is like leaven that a woman took and hid in three measures of flour, till it was all leavened."
The New Leaven
The Gospel, the new leaven, once it has been mixed into the human race (that is, the dough), will not cease working until the end of the world but will permeate the whole lump of those who are to be saved and will extend to all who are worthy, even if all the gates of hell resist [see Matthew 16:18]. And just as it is impossible for the leaven to be separated from the dough once it has been mixed in with the dough (inasmuch as it has changed the nature of the dough), even so it is impossible for Christians to be snatched from Christ, for Christ is the leaven in them that has been made part of them in such a way that they are one body, one lump, one loaf, one bread, etc. (see Romans 12:3-4; 1 Corinthians 12:12-13; Ephesians 4:6). It is therefore in vain that the devil drives the Church from the world and persecutes it—that is, separates Christ from the faithful, the leaven from the dough. For it is as impossible for the devil to separate Christ from His Church as it is for human beings to separate leaven from dough. The dough has been leavened; the devil shall not separate the leaven from the dough. Even if he cooks or roasts it, or even grills it and burns it, Christ the leaven is in it and shall remain in it until the Last Day so that it may all become thoroughly leavened and none of the dough be left unleavened.
From Annotations on Matthew (Luther's Work 67:206)