This is related to the old law. The New Testament law is different. The old law was much more focused on faith. The new law is more focused on actual charity of God. Under the old law, God was much more of a mystery. The new law is God revealed fully. The old law allowed implicit faith because it was a foreshadowing of the fully revealed, new law, which requires explicit faith.
None of this has anything to do with BOD. There is no saint or council which promotes implicit faith for salvation. Only modernists do this.
Implicit faith is a conditional theological concept that only exists if "invincible ignorance" exists.
Because no one can define fully what it means to be in "invincible ignorance", this theological hypothesis has been used by modernists to pretend that since we cannot know with certainty it's the same as "everyone can be saved" technically. Then, from that, they hastily generalize based on pure sophistry that anyone who studied logic would easily spot, but not the average emotional person.
However, if someone actually begins to wonder about the conditions for invincible ignorance, it begins to be clear that, they are extremely restrictive. Humans have free will after all, and "through no fault of his own" on the course of an entire lifetime isn't something many people can achieve.
If invincible ignorance exists at all, we can deduce that something similar to the "implicit faith" of the Old Law exists too.
The way "implicit faith" has been defined is to argue that, if someone does not have access to any information or any priest or any christian who could teach him the explicit articles of the faith, and they try their best to obey the natural law, they also have an "implicit faith" in whatever God's will is. Their ignorance of what exactly God's will entails would not be a sign of guilt in the eyes of God.
This excludes the possibility of the Providence of God directly intervening in order to give what is necessary for salvation to the person. Why am excluding this principle?
It is quite simple : if someone knows the Truth, they are also obligated to teach the Truth to others in order to save their souls. If such a thing were possible, we should have seen plenty of historical evidence of people who have never been in contact with priests or monks and who still knew all the articles of the Catholic faith somehow. Do we have any evidence of such a thing ever happening?
If such a thing exists, we should have lots of evidence of prophets who spontaneously began to teach the True Faith, even before the apostles and their disciples could even reach them. Do we have such evidence?
I do not know how exactly this is supposed to work, but at the very least we have Popes who defined the concept and it's taught in the Catechism.
In any case, this concept is inconsequential for most of the human race.