The Pious Predecessors were very particular about upholding the sound Aqeedah, because it brings the Muslims together, and safeguards them against Shirk (polytheism) and deviation. It also establishes justices. The Companions of the Messenger of Allah, may Allah exalt his mention, set an example for future generations to follow in terms of sacrificing their lives defending the Aqeedah, preaching and promoting it.
The Aqeedah makes the Muslims more attached to the Messenger of Allah, may Allah exalt his mention, and his Companions, with whom Allah is pleased. Allah commands the latter generation of Muslims to follow their footsteps, and adhere to their faith, in order to be safe from falling into the traps of deviant sects.
Those who believe and do not contaminate their faith with Shirk; it is those who will enjoy security, and it is they who are the well-guided.
Ibn Kathir commented on this Ayat saying:
This Ayat refers to those who dedicate worship to Allah alone, sincerely without ascribing a partner to Him. They are the well-guided in this world and the secured on the Day of Resurrection. The Prophet, may Allah exalt his mention, said: He who meets Allah ascribing no partner to Him shall be admitted to Jannah.
Shaikul-Islam b. Taymiyyah said in his comment on this hadeeth:
This applies to him who professes it, and dies while believing in it. The promise in this hadeeth is conditional, that is, to profess it sincerely from his heart, truthfully and certainly, because the essence of Tawheed makes the soul attracted to Allah, the Exalted.
Hence, he who testifies sincerely that there is no true god except Allah, he shall be admitted to Jannah. This can be achieved by repenting sincerely for all sins. If one dies in this condition he shall certainly receive that promise.
It is authentically reported that Allah will finally remove from the Fire anyone who professed the Testimony of Faith, and anyone in whose heart is a grains weight or a mustard seeds weight or an ants weight of Iman.
There are other authentic traditions that promise deliverance to anyone who professes the Testimony of Faith, provided he does so sincerely. Sincerity and strong faith efface sins as daylight effaces the darkness of night.