Answer
Salam Dear Brother/Sister,
Thank you for contacting About Islam with your question.
First of all, I request you to consider what Allah Himself has said in the Quran about His Mercy:
1.{Say: “Unto whom belongs all that is in the heavens and on earth?” Say: “Unto God, who has willed upon Himself the law of grace and mercy […]} (Quran 6:12)
While being the Creator, Owner, and Sustainer of everything in the heavens and earth, Allah Almighty has willed upon Himself the task of showing compassion and mercy to His creatures.
2. {Verily, God does not wrong [anyone] by as much as an atom’s weight; and if there be a good deed, He will multiply it, and will bestow out of His grace a mighty reward.} (Quran 4:40)
So, there is no question of Allah being unjust to anyone in the least degree. And what is more, He will multiply any good that is found in us and give us out of His bounty a mighty reward.
The verses you have quoted should be read in context. Let us go to Surah An’am verses 38-41:
{[…] There is no beast that walks on earth and no bird that flies on its two wings which is not [God’s] creature like yourselves: no single thing have We neglected in Our decree. And once again: Unto their Sustainer shall they [all] be gathered. And they who give the lie to Our messages are deaf and dumb, in darkness deep. Whomever God wills, He lets go astray; and whomever He wills, He places upon a straight way. Say: “Can you see yourselves invoking any but God when God’s chastisement befalls you [in this world], or the Last Hour comes upon you? [Tell me this,] if you are men of truth!” Nay, but it is Him alone that you will invoke – whereupon He may, if He so wills, remove that [ill] which caused you to call unto Him; and you will have forgotten all that to which you [now] ascribe divinity side by side with Him.} (Quran 6:37-41) [Translator: Muhammad Asad]
In these verses, Allah speaks of humans in general. He is referring to a context when His punishment befalls us or when we are aware of the impending Last Hour.
And even if it is a polytheist who calls on Allah in that final distress, Allah, if He so wills, will remove that distress which caused them to call upon Him.
But it is not mentioned there (as you supposed) that such polytheists will be blessed with the ultimate reward of heaven. For further light on this we may read the verses that immediately follow the above verses:
{And, indeed, We sent Our messages unto people before your time, [O Prophet,] and visited them with misfortune and hardship so that they might humble themselves: yet when the misfortune decreed by Us befell them, they did not humble themselves, but rather their hearts grew hard, for Satan had made all their doings seem goodly to them. Then, when they had forgotten all that they had been told to take to heart, We threw open to them the gates of all [good] things until – even as they were rejoicing in what they had been granted – We suddenly took them to task: and lo! they were broken in spirit; and [in the end,] the last remnant of those folk who had been bent on evildoing was wiped out. For all praise is due to God, the Sustainer of all the worlds.} (Quran 6:42-45)
According to Muhammad Asad, the above verses illustrate, “a phenomenon well known in history: namely, the inevitable social and moral disintegration of communities which have lost sight of spiritual truths.” [The Message of the Quran- Note (Number 35) to verse 6:45]
The only way for polytheists to attain salvation from their state is to reject the false gods and idols they worship, and accept Allah Almighty as the one and only God of the universe.
Without such a change of heart, which naturally necessitates a change of their life too, they cannot attain real felicity and success as Allah promises in several verses of the Quran:
{Behold, [only] those who attain to faith and do righteous deeds and humble themselves before their Sustainer – [only] they are destined for paradise, and there shall they abide.} (Quran 11:23)
{Verily, those who attain to faith and do righteous deeds shall have gardens of bliss.} (Quran 31:8)
From the Islamic point of view, a deed becomes good or bad, when it is in conformity with the eternal values enshrined in the Books of God. No human or group of humans can arbitrarily decide good and bad.
As God is the source of all good, the first condition for an action to be valued as good is that it should be directed toward God. So, a good person does good expecting a reward from God.
That is to say, a really good action is motivated by a desire to fulfill the will and commandment of God.
Let us consider the case of the atheists and polytheists: An atheist does not accept the existence of God; he does not pray to a God who is simply ‘not there’. So he does not want any reward from God. And since he is not prepared to receive any reward from God, he doesn’t get it.
The polytheists on the other hand believe in a number of false gods; or a number of entities who are imagined to possess ‘godly power’, who could at best be Allah’s creations like humans or angels. These beings have no power to grant prayers or reward supplicants.
The polytheists approach these false gods expecting rewards; and they do not get them. Neither do they get rewards from God Almighty (except as part of the general scope of Divine Mercy) as they do not ask Allah for rewards, and are not ready to receive rewards from Allah.
In short the polytheists through their false belief would elevate Allah’s creations to the status of the Creator, and in vain expect rewards. So their efforts too are wasted.
We understand from the Quran that Allah accepts the deeds of a person who not only believes in Him, but also expects a reward from Him. Neither the polytheists nor the atheists qualify for rewards from Allah.
The verses quoted above refer to the overarching Mercy of Allah Almighty that extends to all His creatures.
As for Jews and Christians, Islam considers them as People of the Book, namely those who have already received Allah’s Guidance through Revelation given to Moses and Jesus.
The Quran appeals to them to return to the original teachings of God as given in their Books, namely the Torah and the Gospel. And if they sincerely do so, they will find there the prophecy about the Last Prophet who completed the Guidance of God for mankind.
And once this is acknowledged, the only reasonable course open before the People of the Book is to follow Muhammad, peace be upon him.
Please continue feeding your curiosity, and find more info in the following links:
https://aboutislam.net/reading-islam/understanding-islam/5-proofs-that-the-quran-is-the-word-of-god/