Why are the principles of religion (Usool al-deen) only five?
The beliefs in Islam are surely more than the five famous principles. For example, the Almighty God in the Quran states: The Messenger believes in what is revealed to him from His Cherisher (Allah), and (so do) all the believers, they believe in Allah and His angels, and His Scriptures and His messengers… (2:285). As you can see, belief in the angels is not mentioned in the Five Principles, though we have to believe in them. Also, in the beginning of Surah al-Baqara it is mentioned that the pious people are those who believe in the Unseen, which includes belief in the existence of the Jinn too. Yet again, believing in them is not mentioned in the Five Principles.
The reason we only highlight those five articles of faith is because belief in Allah (Tawheed), and the Last Day (the Resurrection) have been the most common teachings of all the prophets. Then, believing in them requires belief in the Messengers who brought those messages to man, thus belief in the Prophets is also an article of faith.
The other two, i.e. justice and Imamat, are added by us Shi’a because the Ash’aris have a wrong interpretation of divine Justice, so in order for us to correct that, we highlighted it in our articles of faith to correct their mistakes. Also, because Imamat is the main gate of our access to prophetic knowledge, and the most important historical debates and divisions in Islam have been on the issue of Imamat, we have highlighted it in our articles of faith.
In short, the main three articles of faith are the most logical issues of belief, and the other two are added due to some historical necessities. Otherwise, our articles of faith are not limited to those five issues.
Answered by: Sheikh Mansour Leghaei