Does the heart's speech increase and decrease, and what is the evidence for that?

The saying of the heart is firm belief and conviction in Allah, His angels, His books, His messengers, and the Last Day. This includes faith in everything the Messenger of Allah, peace and blessings be upon him, brought. This conviction increases and decreases due to people's varying knowledge of the details of what they believe in, their varying levels of its stability in their souls and their remembrance of it, and their varying strengths and weaknesses in conviction.