So this book called al-Uṣūl al-Thalāthah is from the best books of ʿaqīdah (creed) for the seeker of knowledge who is at the beginners level, or at an intermediate level. That means that no one from amongst the students of knowledge is without need of it for the explanation of the correct ʿaqīdah and whatever opposes it. Rather, no one from amongst the Muslims under obligation is free of need from it. So it is built to be memorised and have its meaning understood, and it is also befitting of being taught by the teachers and Scholars, not to mention that the issue of ʿaqīdah discussed herein are of an introductory nature, with respect to the knowledge of the Islamic ʿaqīdah, which should be studied before every other book.
So the Prophets and Messengers, and whomsoever called with their call, continued to follow the call to the Religion of Islām, and to act upon it, and leave off everything else other than it, until the messages were completed with the message of the Prophet (ﷺ).