that doesnt make any sense. Let me put in a very simply way.
Hazrat Muhammad PBUH gave us the good news about Imam Mahdi AS's appearance.
In Quran it is said to obey Allah and His Messenger, whoever disobeys His messenger, is disobeying Allah.
You not accepting Imam Mahdi's appearance is disobeying Prophet Muhammad PBUH, which is disobeying Allah.
If you had read the thread from beginning, the whole discussion is about, if Rasoolullah pbuh did actually say anything about Mahdi or not.