Salam
The Quran has a verse to polytheists that believed in God (but equated others on par with him):
فَلِذَٰلِكَ فَادْعُ ۖ وَاسْتَقِمْ كَمَا أُمِرْتَ ۖ وَلَا تَتَّبِعْ أَهْوَاءَهُمْ ۖ وَقُلْ آمَنْتُ بِمَا أَنْزَلَ اللَّهُ مِنْ كِتَابٍ ۖ وَأُمِرْتُ لِأَعْدِلَ بَيْنَكُمُ ۖ اللَّهُ رَبُّنَا وَرَبُّكُمْ ۖ لَنَا أَعْمَالُنَا وَلَكُمْ أَعْمَالُكُمْ ۖ لَا حُجَّةَ بَيْنَنَا وَبَيْنَكُمُ ۖ اللَّهُ يَجْمَعُ بَيْنَنَا ۖ وَإِلَيْهِ الْمَصِيرُ | So summon to this [unity of religion], and be steadfast, just as you have been commanded, and do not follow their desires, and say, ‘I believe in whatever Book Allah has sent down. I have been commanded to do justice among you. Allah is our Lord and your Lord. Our deeds belong to us and your deeds belong to you. There is no quarrel between us and you. Allah will bring us together and toward Him is the destination.’ | Ash-Shura : 15
And said to people of the book, which can agree on more than that:
قُلْ يَا أَهْلَ الْكِتَابِ تَعَالَوْا إِلَىٰ كَلِمَةٍ سَوَاءٍ بَيْنَنَا وَبَيْنَكُمْ أَلَّا نَعْبُدَ إِلَّا اللَّهَ وَلَا نُشْرِكَ بِهِ شَيْئًا وَلَا يَتَّخِذَ بَعْضُنَا بَعْضًا أَرْبَابًا مِنْ دُونِ اللَّهِ ۚ فَإِنْ تَوَلَّوْا فَقُولُوا اشْهَدُوا بِأَنَّا مُسْلِمُونَ | Say, ‘O People of the Book! Come to a common word between us and you: that we will worship no one but Allah, that we will not ascribe any partner to Him, and that some of us will not take some others as lords besides Allah.’ But if they turn away, say, ‘Be witnesses that we have submitted [to Allah].’ | Aal-i-Imraan : 64
Since people of the book agree only God should be worshiped and we should not obey our leaders while disobeying God as that would be metaphorically taking them as Lord, that's a working point.
For atheists, I would say the starting point would be, to feed the poor and strive for justice for the hungry and oppressed or help free the captives and justice in general. The Quran says if people are good, God will reward their goodness.
When people ask the wicked what put you in hell they say:
We use to not pray/connect.
And We use to deny the day of judgment
And we did not enjoy feeding the poor
And we use to vainly talk/act with the vain talkers/doers.
The Quran by this is saying, pick any of these and do the opposite, and you won't be in hell.
In another verse it says:
وَأَنْزَلْنَا إِلَيْكَ الْكِتَابَ بِالْحَقِّ مُصَدِّقًا لِمَا بَيْنَ يَدَيْهِ مِنَ الْكِتَابِ وَمُهَيْمِنًا عَلَيْهِ ۖ فَاحْكُمْ بَيْنَهُمْ بِمَا أَنْزَلَ اللَّهُ ۖ وَلَا تَتَّبِعْ أَهْوَاءَهُمْ عَمَّا جَاءَكَ مِنَ الْحَقِّ ۚ لِكُلٍّ جَعَلْنَا مِنْكُمْ شِرْعَةً وَمِنْهَاجًا ۚ وَلَوْ شَاءَ اللَّهُ لَجَعَلَكُمْ أُمَّةً وَاحِدَةً وَلَٰكِنْ لِيَبْلُوَكُمْ فِي مَا آتَاكُمْ ۖ فَاسْتَبِقُوا الْخَيْرَاتِ ۚ إِلَى اللَّهِ مَرْجِعُكُمْ جَمِيعًا فَيُنَبِّئُكُمْ بِمَا كُنْتُمْ فِيهِ تَخْتَلِفُونَ | We have sent down to you the Book with the truth, confirming what was before it of the Book and as a guardian over it. So judge between them by what Allah has sent down, and do not follow their desires against the truth that has come to you. For each [community] among you We had appointed a code [of law] and a path, and had Allah wished He would have made you one community, but [His purposes required] that He should test you in respect to what He has given you. So race in all good works. To Allah shall be the return of you all, whereat He will inform you concerning that about which you used to differ. | Al-Maaida : 48
Said elsewhere:
لَيْسُوا سَوَاءً ۗ مِنْ أَهْلِ الْكِتَابِ أُمَّةٌ قَائِمَةٌ يَتْلُونَ آيَاتِ اللَّهِ آنَاءَ اللَّيْلِ وَهُمْ يَسْجُدُونَ | Yet they are not all alike. Among the People of the Book is an upright nation; they recite Allah’s signs in the watches of the night and prostrate. | Aal-i-Imraan : 113
يُؤْمِنُونَ بِاللَّهِ وَالْيَوْمِ الْآخِرِ وَيَأْمُرُونَ بِالْمَعْرُوفِ وَيَنْهَوْنَ عَنِ الْمُنْكَرِ وَيُسَارِعُونَ فِي الْخَيْرَاتِ وَأُولَٰئِكَ مِنَ الصَّالِحِينَ | They have faith in Allah and the Last Day, and bid what is right and forbid what is wrong, and they are active in [performing] good deeds. Those are among the righteous. | Aal-i-Imraan : 114
وَمَا يَفْعَلُوا مِنْ خَيْرٍ فَلَنْ يُكْفَرُوهُ ۗ وَاللَّهُ عَلِيمٌ بِالْمُتَّقِينَ | Whatever good they do, they will not go unappreciated for it, and Allah knows well the Godwary. | Aal-i-Imraan : 115
And else where said:
"Whoever believes in God, God guides his heart"
So there all sorts of doors to heaven.
A door is to not take idols among humans and seek truth. Another door, is to seek justice and the heart will be soft for truth if we are humble enough to take side of oppressed. If we care about hungry, another way to soften heart and make it luminous.
Allah is the Goal but there is all sorts of avenues and doors that open the heart to guidance.
Another door is to simply pray to God humbly for guidance. Another is to reflect over the creation of God.
These are all doors.
At the end, it's not about intellectually knowing the truth. Its about having a heart that is luminous, soft, and good.
