Allah says: “Then set your face upright for religion in the right state-- the nature made by Allah in which He has made humanity. There is no alteration of Allah's creation; that is the right religion, but most people do not know.” [Sūrah Rūm: 30]

Scholars have disagreed about the meaning of “There is no alteration of Allah's creation”. They express two opinions about it:
1. It is a prohibition. Ibn Jarīr al-Tabarī interprets it in this way. He says: “It means: Do not alter Allah’s religion that he made to be the innate nature of His servants. His is the opinion of more than one commentator, who do not mention any other interpretation.”

2. Ibn Ishāq states the view that it is a declarative statement, and that it is on its literal meaning. It means that nobody can alter Allah’s creation.
This second interpretation is more correct. What it entails is that the instinctive nature that Allah creates as the essential character of humanity is unalterable. In short, everyone is born upon the same instinctive nature. No one is created at variance to the innate natural way.

This does not mean that people cannot later on change the nature they were first created upon. We know from the hadīth that they can.

Abū Hurayrah relates that the Prophet (peace be upon him) said: “Every child is born upon the natural way. Then its parents make it a Jew, a Christian, or a Zoroastrian. Just like an animal gives birth to a compete animal. Do you ever see one that is born amputated?” [Sahīh al-Bukhārī (1385, 4775)]

Then Abū Hurayrah read Allah’s words: “the nature made by Allah in which He has made humanity”

We see here that the Prophet (peace be upon him) described the change that can later happen to a person’s natural disposition by comparing it to the amputation or gelding of an animal.

Likewise, we read where Allah says, conveying to us Satan’s threat: “And I will command them, and they will change Allah’s creation.” [Sūrah al-Nisā’: 119] Here, again, the change refers to alterations of the physical body.

Therefore, Allah has given people the ability to change their own natures – the natures He created them upon – by His will and leave. However, as for the essential nature that a human being is initially created upon, no one but Allah has power to affect it, and Allah does not create any human being except upon the natural way: “There is no alteration of Allah’s creation.”

Allah is not saying that the creation cannot be changed, but rather that the nature upon which He created and will create each and every human being will never vary. No alternative nature will ever be bequeathed upon a person at the time of birth. However, if someone changes that nature afterwards, this is another matter. At the time of birth, there was never any alteration in how Allah created each person.

People can change their faith of their own accord. Allah has decreed that they will have the ability to do so. However, no one was born upon an innate disposition for unbelief. We can see from the verse that Allah praises our natural disposition and calls us to remain upon it or to return to it: “Then set your face upright for religion in the right state-- the nature made by Allah in which He has made humanity.”

This nature, therefore, can never be a predisposition for unbelief or falsehood. Allah only commands what is right and good.

[excerpted from: Ibn Qayyim al-Jawziyyah, Shifā’al-Alīl.]