What Nichiren meant by they "Knew about it in their hearts"

Nichiren discusses past masters often referring to their knowledge of Myoho Renge Kyo as they "knew it in their hearts".

What does this mean?

The significance lies in the title of the Lotus Sutra itself.

What does the title signify?

The Pure Dharma of Shakyamuni Buddha. Which Shakyamuni?

Essentially Nichiren is saying that all masters have realized Shakyamuni's enlightenment. However the time was not right for them to propagate it as such.

In the later day of the law, if we look among the people today, what separates us as a people? What contributes to our self imposed suffering? Why is Buddhism, the good medicine the Buddha left for all sentient beings in decline?

Could the lack of such recognition in the hearts and minds of people today somehow contributing to this division?

How does one manifest such an understanding for others when the Daimoku is consistently slandered by doubt?

Is doubt is the last refuge of the Devil of the 6th Heaven?