Reasons for the Decline - Muslims are Responsible
Islam has not declined - The Muslims have, they do not practice Islam any more, and so it Glory and Goodness remains un- achievable, elementary!!!
"If fornication should become widespread you should know that this has never happened without God sending new diseases to the people which their forbears didn't suffer from . If people should begin to cheat in weighing out goods then know that this has never happened without drought and famine befalling the people, and their rulers oppressing them. If people should withhold Zakat then you should realize that this has never happened without the rain being stopped from falling, and were it not for the animal's sake it would never rain again. If people should break their covenant with God and His Messenger you should realize that this has never happened without God sending an enemy against them to take some of their possessions by force. If the leaders do not govern according to the Book of God, you should realize that this has never happened without God dividing them into groups and making them fight one another."
Allaah (subhaanahu wa ta’aala) said as we have seen that “Verily! Allaah will not change the good condition of a people as long as they do not change their state of goodness themselves” and the scholars of tafseer have explained that this verse applies both ways. So due to sin and transgression and innovation.
Allaah will change the good
condition of a people to a bad condition, and vice versa due to
their returning to Tawheed, Sunnah and Tazkiyah Allaah will change
the bad condition of a people to a good condition. This
understanding is further reinforced by Allaah’s saying,
“Allaah has promised those among you who believe, and do righteous
good deeds, that He will certainly grant them succession to (the
present rulers) in the earth, as He granted it to those before them,
and that He will grant them the authority to practice their
religion, that which He has chosen for them (i.e. Islaam). And He
will surely give them in exchange a safe security after their fear
(provided) they (believers) worship Me and do not associate anything
(in worship) with Me. But whoever disbelieved after this, they are
the Faasiqoon (rebellious, disobedient to Allaah). Soorat an-Noor
(24:55)
The Messenger of Allaah (sallAllaahu ‘alaihi wasallam) clearly
stated that the humiliation of the Ummah would only be lifted when
it returned to its deen, “then Allaah will permit your humiliation
and He will not remove it from you, until you return to your
Religion.”7 He did not say ‘when you change your corrupt rulers’.