Loyalty and Renunciation, what is it?

It Is one of the basic principles of Tawheed (affirmation of the oneness of Allah)

Allah, may He be exalted, says (interpretation of the meaning):
“O you who have believed, do not take the Jews and the Christians as allies. They are [in fact] allies of one another. And whoever is an ally to them among you - then indeed, he is [one] of them. Indeed, Allah guides not the wrongdoing people.

So you see those in whose hearts is disease hastening into [association with] them, saying, ‘We are afraid a misfortune may strike us.’ But perhaps Allah will bring conquest or a decision from Him, and they will become, over what they have been concealing within themselves, regretful.
And those who believe will say, ‘Are these the ones who swore by Allah their strongest oaths that indeed they were with you?’ Their deeds have become worthless, and they have become losers.

O you who have believed, whoever of you should revert from his religion - Allah will bring forth [in place of them] a people He will love and who will love Him [who are] humble toward the believers, powerful against the disbelievers; they strive in the cause of Allah and do not fear the blame of a critic. That is the favor of Allah; He bestows it upon whom He wills. And Allah is all-Encompassing and Knowing.

Your ally is none but Allah and [therefore] His Messenger and those who have believed - those who establish prayer and give zakah, and they bow [in worship].
And whoever is an ally of Allah and His Messenger and those who have believed - indeed, the party of Allah - they will be the predominant”
[al-Ma’idah 5:51-56]

Ahmad (22132) narrated from Mu‘adh that he asked the Messenger of Allah (blessings and peace of Allah be upon him) about the best of faith, and he said: “The best of faith is to love for the sake of Allah and hate for the sake of Allah, and to keep your tongue busy in mentioning Him.” Shu‘ayb al-Arna’oot said: It is saheeh because of corroborating evidence.
At-Tabaraani narrated from Ibn ‘Abbas (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “The strongest bond of faith is taking believers as allies and friends for the sake of Allah and to regard disbelievers as enemies for the sake of Allah, to love for the sake of Allah and hate for the sake of Allah, may He be glorified and exalted.” Classed as saheeh by al-Albani in Saheeh al-Jaami‘ as-Sagheer, no. 2539.

Al-Nu’man ibn Bashir reported: The Messenger of Allah, peace and blessings be upon him, said, “The parable of the believers in their affection, mercy, and compassion for each other is that of a body. When any limb aches, the whole body reacts with sleeplessness and fever.”
Source: Ṣaḥīḥ al-Bukhārī 6011, Ṣaḥīḥ Muslim 2586
Grade: Muttafaqun Alayhi (authenticity agreed upon) according to Al-Bukhari and Muslim

Ahmad (22002) narrated from ‘Ubaadah ibn as-Saamit that the Messenger of Allah (blessings and peace of Allah be upon him) said: “[Allah says:] ‘My love is due for those who love one another for My sake; My love is due for those who visit one another for My sake; My love is due for those who help one another for My sake; My love is due for those whose hearts are free of grudges and who uphold ties with one another for My sake.’”
Classed as saheeh by al-Albaani in Saheeh al-Jaami‘ (4321)

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: What is loyalty and disavowal (al-wala’ wa’l-bara’)?

Answer: Disavowal and loyalty for the sake of Allah, may He be glorified, means that a person disavows everything that Allah disavowed, as He, may He be glorified and exalted) says (interpretation of the meaning):

“There has already been for you an excellent pattern in Abraham and those with him, when they said to their people, ‘Indeed, we are disassociated from you and from whatever you worship other than Allah . We have denied you, and there has appeared between us and you animosity and hatred forever’”
[al-Mumtahinah 60:4].

So every believer must disavow every polytheist and disbeliever. This is with regard to people.
Similarly, the Muslim must disavow every deed that is not pleasing to Allah and His Messenger, even if it does not constitute disbelief, such as evildoing and sin, as Allah (may He be glorified, says (interpretation of the meaning):
“but Allah has endeared to you the faith and has made it pleasing in your hearts and has made hateful to you disbelief, defiance and disobedience. Those are the [rightly] guided”
[al-Hujuraat 49:7].

Al-Nu’man ibn Bashir reported: The Messenger of Allah, peace and blessings be upon him, said, “The parable of the believers in their affection, mercy, and compassion for each other is that of a body. When any limb aches, the whole body reacts with sleeplessness and fever.”
Source: Ṣaḥīḥ al-Bukhārī 6011, Ṣaḥīḥ Muslim 2586
Grade: Muttafaqun Alayhi (authenticity agreed upon) according to Al-Bukhari and Muslim