What is the fastest and greatest act of worship, deed, or utterance in obtaining forgiveness for sins? Does uttering "Astaghfirullah" (I seek forgiveness from Allah) forgive old and new sins without remembering them? Does uttering "Subhan Allah" (Glory be to Allah) 100 times forgive a thousand sins? And what is the effect of new sins on repentance from major sins?

To erase all sins, a general repentance is required that includes all sins, those mentioned and those not mentioned. Seeking forgiveness (Istighfar) alone is a supplication that may or may not be answered. However, seeking forgiveness for a specific sin coupled with repentance for it will expiate that sin. General repentance erases sins, no matter how great they are. Good deeds expiate minor sins, not major ones, such as remembrance (dhikr) by the tongue. The best way to expiate sins is sincere repentance (Tawbah Nasuh). If you do not remember the sin, general repentance is sufficient, and all good deeds and seeking forgiveness are expiations for sins. The virtue of glorification (Tasbeeh) is mentioned in the Hadith: "Whoever glorifies Allah one hundred times, one thousand good deeds will be written for him, or one thousand sins will be removed from him." The expiation of sins and their extent is up to Allah; therefore, one must multiply good deeds, strive to fulfill the conditions of repentance, and have a good opinion of Allah.