Fatima comes with a two-fold contradiction.
The first is obvious: Conversion of Communists.
The second is more subtle: Conversion of Secular Humanists.
Marx himself said of Communism (marxism) that it and secular Humanism are the same: Both deny the existence of a God.

This is why Fatima has a two-fold reason for being.

The battle is continually waged by those entities that are against God. Mary provides both the grace as mediatrix of graces, and also the solidarity so needful for Catholics who passionately want to build a community of believers, whose Catholic values they share. Values are not always the same. What Catholics value is very different to what atheists value. Christian tradition continues with constancy, perseverance, and attention to detail. In true Catholic tradition, Catholicism invokes Mary inclines to her aid and honours her like no other religion.
The temptation for every Catholic is to see no difference between good and evil. It is Mary’s wisdom which provides, and this thwarts evil from taking root in innocent minds, with assistance in every trial.
The presence of Marian movements (e.g. Legion of Mary, Mariapolis etc. ) has one effect: Becoming the leaven in the bread. Without leaven, the bread does not rise.
This is the purpose of gathering together in Mary. It is to become a leaven in the church.