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Allaah says
"Verily, those who say 'Our Lord is Allaah(alone)', and then they stand firm, on them the angels will descend (at the time of their death) saying: "Fear not, nor grieve! But receive glad tidings of paradise which you have been promised! We have been your friends in this life of this world and are(so) in the Hereafter. Therein you shall have(all) that your inner-selves desire, and therein you shall have (all) for which you have been promised. An entertainment from(Allaah), the Oft-Forgiving, the Most Merciful" ~Suratu Fussilat vs 30-32.
Shaykh Uthaymeen (RahimahuLlaah) regarding this verse, says
"Istiqama" means to adhere firmly to the Islamic injuctions. As commanded by Allaah- The Exalted, Together with having Ikhlaas(complete sincerity to Him).

In suratu Hood vs 112, Allaah said:
"So stand (ask Allaah to make you(Muhammad)) firm and straight (on the religion of Islamic Monotheism) as you have made commandeed."
This verse generally addresses Rasul(sala Llaahu alayhi was salam) and his Ummah(his followers) .

So it is upon each one to have Istiqama as commanded by Allaah azza wa jal and not change his/her religion, nor to increaser or decrease in the religion.

So Allaah says about this people:
"Verily, those who say: "Our Lord is Allaah (Alone)"
This means: they Proclaim that Allaah created us, and He is our Lord and the disposer of our affairs and we are sincere to Him.
Then Allaah says: "And then they Stay firm" (that is by observing the legislation of Allaah)
:"On them will the angels descend" meaning at the time of their death saying to them "Fear not nor grieve"(Meaning; fear not what you will face in the Next life, Nor be grieved over your past affairs I.e things you've left behind you in this life.
Allaah then says: "But receive the glad tidings of paradise, which you have been promised"

...To be continued Inshaa Allaah!