What is the response to the doubt of abandoning action because the divine decree has already been fulfilled?

Question:
A questioner says: Satan and his evil companions confuse some of the youth regarding the issue of qadar (divine decree), claiming that there is no benefit in holding fast to the religion as long as the matter has already been fulfilled. We ask for your guidance, may Allah reward you with the best reward?

Answer:
This doubt occurred in the past during the time of the Prophet (peace and blessings be upon him); it occurred with the Companions (may Allah be pleased with them) . When the Messenger (peace and blessings be upon him) informed them and said: ما منكم من أحد إلا وقد عُلِمَ مقعده من الجنة ومقعده من النار، قالوا: يا رسول الله! ففيمَ العمل حينئذ؟! قال: اعملوا فكل ميسر لما خلق له؛ أما أهل السعادة فيُيَسَّروا لعمل أهل السعادة، وأما أهل الشقاوة فيُيَسَّروا لعمل أهل الشقاوة، ثم تلا قوله سبحانه: فَأَمَّا مَنْ أَعْطَى واتَّقَى ۝ وصَدَّقَ بِالْحُسْنَى ۝ None of you is there but that his place in Paradise and his place in the Fire has been known. They said: O Messenger of Allah! Then what is the work for in that case? He said: Work, for everyone is facilitated for what he was created for; as for the people of happiness, they are facilitated for the deeds of the people of happiness, and as for the people of misery, they are facilitated for the deeds of the people of misery. Then he recited His saying, Exalted is He: But as for he who gives and fears Allah and believes in the best reward, and as for he who is stingy and considers himself self-sufficient and denies the best reward then We will make difficult for him the difficult path [Al-Layl:5-10].
This makes it clear to us that qadar (divine decree) is already fulfilled, and that Allah knows everything and has counted everything ; but He has established causes for this and causes for that. There are causes for good, and causes for evil; there are deeds for Paradise, and deeds for the Fire. So whoever is among the people of happiness, Allah guides him to the deeds of the people of happiness and helps him, and whoever is among the people of misery, he is made easy for the deeds of the people of misery... and there is no power and no strength except by Allah.
So the believer must be earnest in the work that he knows is good and is among the deeds of the people of happiness. The believer must be earnest in it, hasten to it, increase in it, and beware of the deeds that Allah has made among the deeds of the people of misery, and distance himself from them, and make people averse to them.