Imam al-Barbahaaree [d. 329 AH], may Allah have mercy upon him, said:
If something from innovation becomes apparent to you in a person, beware of him, because that which you are unaware of is greater than that which is apparent to you.
Shaykh Ahmad an-Najmee [d. 1429 AH], may Allah have mercy upon him, commented on this passage by saying:
The intent of this passage is to drive you away from those who have innovation with them and to inform you that the people of innovation try to conceal and hide their innovations.
However, the people of the Sunnah avoid being secretive about affairs of the religion, and they view concealment in the affairs of religion to be from the way of the people of innovation.
‘Umar Ibn ‘Abdul-‘Azeez [d. 101 AH], may Allah have mercy on him, said:“If you see a people speaking confidentially about the religion, excluding the general population, know that they are laying the foundation for misguidance.” (1)
For this reason, the people of the Sunnah do not conceal affairs pertaining to the religion. They stay way from this type of behavior, and they consider it to be from the ways of the people of innovation. (2)
There is a subtle benefit and principle that the people of the Sunnah mention pertaining to the secretive nature of the people of innovation. Al-‘Awzaa’ee [d. 157 AH], may Allah have mercy on him, said: “Whoever conceals his innovation from us, his associates will not be hidden from us.” (3)
(1) See “The History of ‘Umar Ibn ‘Abdul-Azeez by Ibn al-Jawzee,” pg. 54
(2) Ahmad an-Najmee, An Explanation of Sharh as-Sunnah, 224-225
(3) Ibn al-Battah, al-Ibaanah, 2/476