Question

Why is declaring faith the first of the five pillars of Islam?

Answer 1

The philosophy is simple that no one can start the journey towards being a Muslim unless he professes the basic principles mentioned in the  "Declaration of Faith", which is "“There is none worthy of worship but Allah and Muhammad is His Messenger”.
The declaration consists of the two most fundamental elements of Islamic ideology:
1) Unity of Allah: Declaring that there is only One God and none else is mandatory to be a Muslim.
2) Faith in the prophet hood of Muhammad, the messenger of Islam.