Praise be to Allah.
The ‘holding fast” that is mentioned in the Qur’an is of two types: holding fast to Allah, and holding fast to the rope of Allah.
Allah, may He be exalted, says (interpretation of the meaning):
“And hold firmly to the rope of Allah all together and do not become divided”
[Aal ‘Imraan 3:103]
“and hold fast to Allah. He is your protector; and excellent is the protector, and excellent is the helper”
Imam Ibn al-Qayyim (may Allah have mercy on him) said:
I‘tisaam (holding fast) means adhering to that which will protect you and will ward off from you that which you dread and fear. ‘Ismah means protection and i‘tisaam means seeking protection. Hence fortresses are called ‘awaasim (sing. ‘aasimah) because they give protection and safety.
Well-being and happiness in this world and the hereafter are based on holding fast to Allah and holding fast to His rope. There is no salvation except for the one who adheres to these two means of protection.
With regard to holding fast to His rope, it protects one from going astray.
Holding fast to Him protects one from doom and destruction, for the one who is journeying towards Allah is like one who is travelling on a path towards his intended destination. Hence he needs guidance on the road and he needs to be safe whilst travelling; he cannot reach his intended destination except after attaining these two things.
The guide will protect him from losing his way, and will guide him to the right path; and equipment, physical strength and weapons will help him to be safe from bandits and any other troubles he may encounter on the road.
So holding fast to the rope of Allah will lead him on the path of guidance and help him to follow the guide.
Holding fast to Allah will give him the strength, equipment, weapons and anything else that he will need [to protect himself] on his journey.
Hence the salaf [early generations] used a variety of phrases to explain what is meant by holding fast to the rope of Allah, even though these phrases all refer to the same idea.
Ibn ‘Abbaas said: Adhere to the religion of Allah.
Ibn Mas‘ood said: It is the jamaa‘ah (main body of the Muslims). And he said: You should adhere to the jamaa‘ah, for it is the rope of Allah that He has enjoined us to hold fast to. What you may find of things you dislike when you are with the jamaa‘ah is better than what you may find of things you like when you are scattered.
Mujaahid and ‘Ataa’ said: It means adhering to the covenant of Allah. Qataadah, as-Saddi and many of the scholars of tafseer said: It means adhering to the Qur’an.
Then he said: As for holding fast to Him, it means putting one’s trust in Him, seeking strength with Him, seeking protection with Him, and asking Him to protect the individual, give him strength and ward off harm from him. For the result of holding fast to Him is that He will ward off harm from His slave, for Allah defends those who believe. So He will ward off from his believing slave, if he holds fast to Him, any cause that could lead to problems, and He will protect him from that. So He will protect him from doubts and desires, and from the schemes of his enemies both apparent and hidden, and from the evil of his own self. He will ward off from him the consequences of that which may lead to problems and troubles after the causes thereof are all present, commensurate with the strength with which he holds fast to Him, so that the causes of troubles and problems will be removed in his case, and thus Allah will protect him from what could result from that and the causes thereof. He may ward off from him something that He had decreed by means of another decree, and something that He willed by means of something else that He wills, thus protecting him from it. End quote from Madaarij as-Saalikeen (1/457-460).
We also advise you to refer to the following material [in Arabic]:
al-I‘tisaam bi’l-Kitaabi wa’s-Sunnah: Asl as-Sa‘aadah fi’d-Dunya wa’l-Aakhirah wa Najaat min Mudillaat al-Fitan by Shaykh Dr. Sa‘eed ibn ‘Ali ibn Wahf al-Qahtaani (may Allah have mercy on him).
And Allah knows best.