Many non-Muslims get enraged when they hear that only Followers of Islam will enter heaven. That is absolutely normal behavior, and there is nothing wrong with it. If I was in their shoes, I would probably do the exact same thing, and start to patronize Islam and Muslims on the whole. But the fact of the matter is, there is a reason why Followers of Islam say that they are the only ones who will enter Heaven. I will try to explain it over here as clearly as possible, and I'll hope that my non-Muslim brothers and sisters will be able to understand and accept the logic behind it, inshAllah.
Followers of Islam believe in a single supernatural being governing all creation and call that entity Allah. Unlike other religions, Islam is the only religion where devotion and worship is restricted to one and only one God. Although other religions have similar claims, but there practices and beliefs prove otherwise.
The Qur'an and Ahadeeth teach us that actions are judged by intentions. Allah clearly mentions in the Qur'an that if you do any good deed with the sole intent of pleasing Him then He wills surely reward you for it in the hereafter, whereas if there is any other goal behind it then that deed holds no value in the afterlife and will be recompensed in this world itself.
If you do the kindest action possible with an evil purpose behind it then it is nothing short of a sin. It rather works against than for you, as you initially thought. You might be able to curry favor in this world with that action, but the hereafter will be full of difficulties for you. Thus, an intention can be noble only when its aim is to please Allah, and that's because there can be no good without God. In fact, if you take out "God" from the word "good", all you are left with is an O - which closely resembles the value of nothingness i.e. zero.
There are many righteous and selfless people in this world, but not everyone's actions are pointed at the pleasure of Allah. And so, heaven is a place only for those people who did strive hard in this life in order to please Allah.
You might also say that followers of other religions are also doing a lot of good work in the name of God, what about those people? Like I said before, many religions claim to worship one God, but their beliefs contradict it. The most hated sin is shirk (joining partners in worship with Allah), and those people are committing shirk under the misconception of pleasing God. If they really wanted to know God then they would realize it themselves that anything other than Islam is not leading them solely and only towards Him. And I am not the only one saying it. Every year hundreds of thousands of people are willfully entering the fold of Islam; some of them are even scholars and priests of their previous religion, and they admit it themselves that how they found loopholes in their religion and found peace and truth in Islam.
Every other religion has some element or the other where they are worshiping multiple beings or material beings. Thus, if an action has "I want to please Jesus Christ or any other being" motto behind it then it rather infuriates Allah instead of pleasing Him because of shirk. As a result, the good actions which were committed with a goal different from pleasing Allah are only going to help people in this world. In the hereafter, the only currency that works is noble actions aimed at pleasing the one and only God. People who devoutly follow Islam are the only ones who have that intention and therefore, Allah will reward with them with what He promised.
I hope my explanation was clear cut and easy to understand.
