Is it essential to mention all sins when repenting?

The one who falls into any type of shirk or any other sin does not have to mention each type thereof; rather it is sufficient for him to be sincere towards Allah, may He be exalted, in repenting in general terms, fulfilling all the conditions required for his repentance to be accepted.

Shaykh al-Islam Ibn Taymiyah said in al-Fataawa al-Kubra (5/281):  If a person repents in general terms, this repentance will lead to forgiveness of all his sins, even if he does not call to mind each particular sin, unless there is a reason to bring to mind one or more of his sins in particular, such as if there is one sin which, if he were to bring it to mind he would not repent from it, because he still has a strong inclination towards it, or because he believes that it is good and is not bad. So if there is a sin which, if he were to bring it to mind he would not want to repent from it, it is not included in that repentance. As for the sins which, if he brings them to mind when uttering general words of repentance, he would want to repent from them, then they are included in this general repentance. End quote.

Imam Ibn al-Qayyim (may Allah have mercy on him) said in Madaarij as–Saalikeen (1/283): Hastening to repent from sin is obligatory straightaway, and it is not permissible to delay it. If the individual delays it, he has committed another sin by delaying. So when he repents from the sin, there remains another act of repentance that is required of him, which is repenting from delaying repentance. This rarely crosses the mind of the one who repents: rather he thinks that if he repents from the sin, nothing else is required of him, when there still remains repentance from delaying repentance. Nothing can save a person from that except repenting in general terms, which includes all the sins of which he is aware and those of which he is not aware, for the sins of which he is not aware are more numerous than those of which he is aware. His being unaware of his sins will not excuse him from being brought to account for them, if he is able to seek knowledge, because he is sinning by failing to seek knowledge and act in a proper manner, so the sin in his case is exacerbated. In Saheeh Ibn Hibbaan it is narrated that the Prophet (blessings and peace of Allah be upon him) said: “Shirk in this ummah is more subtle than the sound of an ant’s footsteps.” Abu Bakr said: How can we rid ourselves of it, O Messenger of Allah? He said: “By saying: ‘Allahumma inni a‘oodhu bika an ushrika bika wa ana a‘lam wa astaghfiruka lima laa a‘lam (O Allah, I seek refuge with You lest I associate anything with You knowingly, and I seek Your forgiveness for that of which I am not aware).” This is a request for forgiveness from that which Allah knows is a sin but the individual is not aware of it. In as-Saheeh it is narrated from the Prophet (blessings and peace of Allah be upon him) that he used to offer the following supplication in his prayer: “Allahumm aghfir li khatee’ati wa jahli wa israafi fi amri wa ma anta a‘lamu bihi minni. Allaahumm aghfir li jiddi wa hazli, wa khata’i wa ‘amdi, wa kulla dhaalika ‘indi. Allaahumm aghfir li ma qaddamtu wa ma akhkhartu wa ma asrartu wa ma a‘lantu, wa ma anta a‘lamu bihi minni. Anta ilaahi laa ilaaha illa anta (O Allah, forgive me my mistakes, my ignorance and my transgressing the limits of righteousness in my deeds and whatever You know better than I. O Allah, forgive me the wrongs that I have committed in earnest or in jest, mistakenly or deliberately, for all of that is possible in me. O Allah, forgive my past and future sins, what I have done secretly and openly, and whatever You know better than I. You are my God; there is no god worthy of worship except You).”

According to another hadith: “Allahumm aghfir li dhanbi kullahu diqqahu wa jillahu khata’ahu wa ‘amdahu sirrahu wa ‘alaaniyatahu awwalahu wa aakhirahu (O Allah, forgive me all my sins, small and great, by mistake or deliberate, secret and open, the first and the last of them).” These general and comprehensive words will make a person’s repentance include whatever of his sins he is aware of and those of which he is not aware. End quote.

And Allah knows best.