Regimini militantis ecclesiae is a Latin phrase that means "the regime of the Church Militant." This phrase refers to the organization and governance of the Catholic Church on Earth.
Okay, let's break it down a bit. The Catholic Church is an institution that has been around for over 2,000 years. It has a certain way of doing things and making decisions about important matters. This way of doing things is called the "regime."
But why is it called the "Church Militant?" Well, according to Catholic teaching, the Church is made up of all the faithful (that's all the people who believe in God and follow the teachings of the Church). The Church on Earth is called the "Church Militant" because it is still fighting and struggling to spread the message of Jesus Christ and bring more people to know God. So, when we talk about the "regime of the Church Militant," we're talking about how the Church is organized to help achieve its goal of spreading God's message and bringing people closer to Him.
When we talk about this regime, we're talking about things like how the Church is structured (who's in charge, what roles different people play), how decisions are made (who gets to decide important things and how they make those decisions), and how the Church is run day-to-day (how things get done and how people work together).
So, in summary, regimini militantis ecclesiae is a fancy way of talking about how the Catholic Church is organized and run on Earth to spread God's message and bring people closer to Him.