In The Joy of the Gospel, Pope Francis mentions a distinction between two types of faith. In no. 124 he talks about popular piety:
is it [popular piety] devoid of content; rather it discovers and expresses that content more by
way of symbols than by discursive reasoning, and in the act of faith greater
accent is placed on credere in Deum
than on credere Deum."
What is the difference between them? A note here references St. Thomas on faith.
Briefly, "credere in Deum" is a living faith (what Thomas would call a formed faith). It's a faith that works through love, a faith that is not a dead letter for the person who has it, but one that urges them ever closer to God. It means to believe in God not just in an abstract intellectual way, but with love, with our whole will.
"Credere Deum" instead refers to the content of faith. It's the "what" we believe. That's very important too, and we need to know the content of our faith. But it needs to be completed by the "credere in Deum" that moves us to love God and show our faith in the way we live.
For example, Catholics who believe in what the Church teaches about the Mass and the sacraments, but never go to Mass, have credere Deum but not credere in Deum. In terms of our own spiritual life, the more we can move from credere Deum to credere in Deum, the holier we will become.
So in the context of this part of the letter, I think Francis is saying that authentic popular piety is linked with a real living out of faith. It leads to a loving, dynamic faith, one that doesn't stop at only marking out the limits of what we believe, but leads to a faith that changes our lives.
The reference is to the Summa, II-II, q. 2, article 2.