What is meant by the soundness of creed?

Linguistically, 'aqeedah' is derived from 'aqd' (tying, عقد) and 'jazm' (firmness, جزم). Islamically, it is a firm mental conviction. If it conforms to reality, it is sound; otherwise, it is invalid. The sound 'aqeedah' is that which the Prophet (peace be upon him), his Companions, and the early predecessors adhered to. This is indicated by Allah’s saying: (So if they believe in the like of what you have believed in, then they have been [rightly] guided) and His saying: (And the first forerunners [in Islam] among the Muhajireen and the Ansar and those who followed them with good conduct – Allah is pleased with them and they are pleased with Him). Also, the Prophet (peace be upon him) said: "And they are those who are upon what I and my Companions are upon." And he said: "The best of people are my generation, then those who follow them, then those who follow them."