I am having many doubts regarding Islam, but I want to have true faith. How does one know when he has true faith? Is certainty necessary for it?
First of all you have to come to know that there is a Creator of the universe. You should know that there is nothing that exists without a cause. Anything that beings to exist has a cause. The universe began to exist so it has a cause. That cause must be uncaused otherwise we will reach infinite regression. That cause must be All-Powerful then whatever we human beings can imagine.That cause me intelligent. That cause be Eternal. If you know this from your reasoning you come to know there is a Creator who did this. I didn't not reach yet to explain what is faith yet. Now you have many religions Islam, Hinduism, Christianity, Judaism etc.
Do you believe that the Creator sent Messengers to guide human beings.?if not let's Analyse who is Muhammad S.A.W.
Well he was an illiterate human being who couldn't read and write. But he had morals. From history we come to know that he was not delusional because a delusional message doesn't reach the quarter of the planet.He was not a liar, he never deceived people. He never asked payment for spreading the message despite the fact he was offered many things for not spreading the message. He had status also , he was from Quraish tribe of banu hashim clan. After reasoning we can come to the conclusion he was the last Messenger of Allah. You can doubt but you can't change the fact. Human mind can imagine whatever they want. But through our reasoning and understanding we can reach to the conclusion that Muhammad S.A.W is the last Messenger of Allah and the Creator is only Allah. Allah is the name of the the Creator which means one God.
Regarding Quran we know Muhammad S.A.W claimed that its completely the word of Allah. Plus it contains information which no human being ever knew in the 7th century. For example about universe expanding and embryology.
So we know now that there is a Creator and that Creator is only Allah and that Muhammad S.A.W is the Messenger of Allah and Quran is the word of Allah. If you accept this as true and believe in the seven articles of faith are true and also believe that Islam is 100% true then you are in a right direction.
7 articles of faith are : Belief in Allah Belief in Angels Belief in the Books Allah sent Belief in Messengers/Prophets Belief in the Last Day Belief in Predestination Belief in being raised up again on the Day of judgement
You are a Muslim if you accept Islam is completely true and you say the Shahadahtain that there is no God but Allah and that Muhammad S.A.W is the last and final Messenger of Allah.
Faith comes from knowledge, reasoning and understanding and by accepting the truth.
Also certainity is important.
I hope I have helped you a bit.