Also, do you think someone who doesn't believe in those three stages and doesn't see this truth would his entire faith and actions be in vain?
It depends if he is aware or not. People who reject the journey to God for example, and many people do, are people deceiving themselves about their belief in God.
People who approach the Quran with no want to understand the inner meanings of the Shariah will probably go to the Quran to confirm their desires and not seek the truth.
The book of God is primarily a book dealing with traveling to God. God has given all sorts of parables in there, and has made parallels to them in the Shariah for a purpose.
If a person becomes aware of the emphasis of 7 with the 7 veils or 7 heavens or 6 days and 7th being that of God's establishment on the throne, and yet doesn't put significance to 7 takbeers before Salah with any of that after hearing Imams emphasize on that, they are not allowed to out of desires.
Like wise if a person wishes to dissociate water in wudoo with light of God and any symbolic meaning with respect to the nature of God's Names and humanity, even after becoming aware of the inward dimension, it can be said he hates the mystic link with God that humanity has.
All this cannot be explained except with love of Dunya. Love of Dunya is what causes people to reject knowledge of a higher type.
Also to downplay anything of the Quran, any verse, anything it says, is unacceptable. This is not only out of love of Dunya, but is sheer disbelief, even if a person is unaware of it.
This is because God chose out of infinite words these words. These words are all careful part of a design of a book where each Surah plays a vital role, and where every verse is significant beyond our appreciation.
It's upon us to look into a verse as a treasure box and try to find what we can open up with it.
Not down grade it. Often a person down grades a hadith or a verse, and he becomes far away from the door of light and path to God though he doesn't perceive. He thinks he has faith in God and the last day, but deceives himself.