Introduction

A Muslim who is born and raised in a Muslim country where he consciously and subconsciously absorbs the laws, values and teachings of Islam, grows up into a young person who is aware of the customs of his religion, following its path and is led by its guidance. On the other hand, a Muslim who is born, and brought up in a non-Muslim country demonstrates the influence of that environment very clearly in his thoughts, ideas, behaviour, values, and etiquette unless his Lord helps him.
This un-Islamic influence is seen more in the second generation of those who have migrated to non-Muslim countries.

This was the reason for Islam's view on at-ta'arrub ba'd al-hijra as reflected in many ahadith.
At-ta'arrub ba'd al-hijra literally means "becoming shorn of one's percepts of faith after migrating [to city]," and technically, it means leaving an environment where you could follow Islam and moving to a place where you maybe prone to not following Islam.
Such a migration is counted as one of the major sins.
Abu Basir says that he heard Imam as-Sadiq (a.s) saying: "The major sins are seven: killing a person intentionally; associating someone or something with the Almighty Allah (shirk); wrongfully accusing a married woman of adultery; Knowingly dealing in usury; running away from the battle-field in jihad; at-ta'arrub ba'd al-hijra; causing distress to one's parents [by encroaching on their rights]; and wrongfully acquiring the property of the orphan.
" Then he said, "At-ta'arrub and shirk are one and the same [in severity]." ⁽¹⁾

Ibn Mahbûb narrates that some of our companions wrote through me a letter to Imam al-Hasan al-'Askari (a.s) asking him concerning the major sins.
He (a.s) wrote: "The major sins are the ones for which Allah has threatened with the Hell-Fire; the one who refrains from them, He will forgive his sins if he is a believer.
Those seven which cause [one to burn in Hell Fire] are: killing an innocent person; causing distress to one's parents [by not upholding their rights]; dabbling in usury; at-ta'arrub ba'd al-hijra; wrongfully accusing a married woman of adultery; unlawfully confiscating the property of the orphan; and running away from the battle-field in jihad." ⁽²⁾

Muhammad bin Muslim narrates from Imam as-Sadiq (a.s): "The major sins are seven; intentionally killing a believer; wrongfully accusing a married woman of adultery; running away from the battle-field in jihad; at-ta'arrub ba'd al-hijra; unlawfully confiscating the property of the orphan; dabbling in usury; and every act for which [the punishment of] the Fire has been promised"? ⁽³⁾ 'Ubaydullah bin Zurarah narrates that he asked Imam as-Sadiq (a.s) about the major sins.
The Imam said, "In the book of [Imam] 'Ali, they are seven: disbelieving in Allah; killing a person; causing distress to one's parents; dabbling in usury; unlawfully confiscating the property of the orphan; running away from the battle-field in jihad; at-ta'arrub ba'd al-hijra.
" Then he asked, "So these are the most major of sins?" The Imam replied, "Yes.
" ⁽⁴⁾

Imam ar-Rida (a.s) explained the prohibition of at-ta'arrub ba'd al-hijra as follows: "Since there is the danger that because of at-ta'arrub, he [the immigrant] might abandon [Islamic] knowledge, get involved with the ignorant people, and drift away" ⁽⁵⁾ This, however, does not mean that entering non-Muslim countries is always forbidden.
Other ahadith had described for us the reward of one who visits non-Muslim lands, the reward that every Muslim longs for.
Hammad al-Sindi narrates that he asked Imam as-Sadiq (a.s), "I visit the cities of polytheism [i.e., of the polytheists]; and there are some among us who say that 'if you die over there, you will be raised [in the Hereafter] along with them.
'" The Imam asked me, "O Hammad, when you are over there do you talk about our affair [i.e., our truth] and call [people] to it?" I replied, "Yes.
" The Imam asked me, "When you are in these cities, the cities of Islam, do you talk about our affair and call [people] to it?" I replied, "No." The Imam said, "If you die over there [in the land of the non-Muslims], you will be raised as an ummah by yourself, and there will be light in front of you!" ⁽⁶⁾