If one is born a Muslim, does that person still has to say the Shahada, or he/she won't be considered a Muslim?
No, shahada is obligatory for someone to enter islam, if he/she is born muslim it is not required except if he left islam then to get back the shahada is obligatory.
There are a number of obligatory conditions for a human to be Muslim but five most common ones are:
- Saying Shahadah
- Offering salah five times a day.
- Doing fast in Ramadan for one month a year. Saum
- Visiting Kaaba if in proper conditions by mean of budget at least once in whole life. Hajj
- Giving zakat
Like offering Salah, it is also another obligatory course for being Muslim. However, on the other hand, a person may not be in condition to speak or may not know about this but feels that he/she is Muslim. At that time we should consider that one is a Muslim when he/she accepts what "Shahadah" expression dictates. But we must also search hadith for this.
Understanding the purpose of Shahada explains it's need.
Shahada is literally to bear witness.... when we say that someone who accepts Islam needs to recite the shahada it is to say that this is point in which direction has changed for the individual their actions that demonstrated what they bear witness to with their life have now changed direction and the physical act of announcing their faith is the turning point.
However, is it possible for someone to change their life and live a new set of values without declaring them, and people witnessing that declaration? Absolutely. Once someone is living the values they believe in, they are bearing witness to their faith. We see shahada of a muslim through his hands, his words, his eyes, his feet by all being in compliance with what Allah has asked us to perform.
Therefore, it's not even required that someone recite the shahada outloud to accept Islam, one can live the values of Islam and acknowledge in his heart his role and relationship with the creator to be a Muslim.
We recite the shahada aloud to give us strength, and reaffirm the desires of proximity to God and his true path that is in our hearts.
Hope it helps :-)