It should suffice to ‘legally’ declare or consider someone a Muslim if he/she makes the following declaration:
“I bear witness that there is no god but Allah, and Muhammad (s.a.w.) is His last and final Messenger. I solemnly vow to submit, surrender and yield to all of Allah’s Laws and Commands as given in His last and Final Book, i. e. the Quran. I realize that in the Hereafter, I shall surely be held accountable in Allah’s Court for all of my words and deeds.”
 s.a.w. = sallAllahu alaihi wasallam (peace be upon him)
 The word ‘Muslim’ literally means the ‘one who submits, surrenders or yields to an authority’. In proper sense, the term ‘Al-Muslim’ refers to ‘the one who submits, surrenders or yields to Allah’s Will or Command’. The word ‘Momin’ means ‘a believer’. In proper sense, the term ‘Al-Momin’ refers to ‘the one who believes in the Islamic Ideology or value system, including belief in One God, His Prophets/Messengers, Malaika, Divine Books, and the Hereafter (which is so far hidden from our sight, i.e. al-ghaib) [Al-Quran Surah 2: Verse 177]. Another meaning of ‘Momin’ is ‘the one who gives peace and security’. One of the attributes of Allah is also Al-Momin [Al-Quran Surah 59: Verse 23]. The constitution of the Islamic Republic of Pakistan defines Muslim as “a person who believes in the unity and oneness of Almighty Allah, in the absolute and unqualified finality of the Prophethood of Muhammad (peace be upon him), the last of the prophets, and does not believe in, or recognize as a prophet or religious reformer, any person who claimed or claims to be a prophet, in any sense of the word or of any description whatsoever, after Muhammad (peace be upon him)”.
 The first sentence of the declaration is about the major beliefs; the second is about deeds; and the third is about a sense of accountability. All the other things which may be necessary to include in the definition are covered in the “vow to submit, surrender and yield to all of Allah’s Laws and Commands as given in His last and Final Book, i. e. the Quran”.