4 men are praying. One Muslim man, one Jewish man and one Christian and one non religious man These 4 men ask for the same thing. They all seek for happiness and blessings. I m aware Allah can see through into their hearts, but suppose all these 4 men are sincere and honest men. Even though the non religious man doesn't go to the mosque/church/synagogue, he knows there's 1 God the Creator of the entire universe. Whose prayer will Allah hear and why?
Firstly, Allah hears all four men who supplicate to Him, as He is the all hearing. The matter comes to who does He answer, the answer is if the one supplicating has sincerity and is praying to God, then that person would have his supplication answered. Why, because of Sincerity, and Allah hears all supplication and can answer it.
Further explanation, Allah has said to us Muslimeen that if we call upon Him then He would answer:
وَقَالَ رَبُّكُمُ ادْعُونِي أَسْتَجِبْ لَكُمْ ۚ إِنَّ الَّذِينَ يَسْتَكْبِرُونَ عَنْ عِبَادَتِي سَيَدْخُلُونَ جَهَنَّمَ دَاخِرِينَ
And your Lord says, "Call upon Me; I will respond to you." Indeed, those who disdain My worship will enter Hell [rendered] contemptible.
As for the Jewish, or Christian, or non-religious man, Allah can answer their prayers or He may not. If the Kafir makes Dua' to Allah alone and no one or no thing else, sincerely then Allah will answer His prayer. But this is in regards to worldly things, for instance if a Kafir asks for food or water or some other benefit then he would be given, or if he asks for protection from some harm then he could be given it. A great example are those who go to sea and when a storm comes they sincerely turn to God for protection, so when they get to land safely they disbelieve.