The book is, of course, insufficient to ground one in the Catholic faith, but as a Christian apologetic, it is very good.
Lewis’s own journey to faith is perhaps revealing. He writes in “Surprised by Joy” that he first ceased to be an atheist, and became a theist. As a second step, he became a Christian (Anglican). “Mere Christianity” is a valuable apologetic for those who reject the Christian concept of God. It clears the ground to make them open to considering that God exists, and gives an overview of the Christian world view. From there, like Lewis, they can progress to Christian faith, hopefully to the fullness of that faith in the Catholic Church, either initially or eventually.
Blessings,
Gerry