If Dev was truly a faithful devotee, why did he so quickly accuse Baba of deception instead of trusting that the promise was fulfilled in an unseen way?

Dev's reaction stemmed from human fallibility and a 'veil of Maya' that clouded his perception, as detailed in Chapter 40. The text explains that 'only this much was fixed in Dev’s mind, and the state of delusion strengthened' when the Sanyasi appeared. Dev himself later regretted his lack of insight, blaming the 'pride of my knowledge.' His confusion was compounded by the fact that the Sanyasi had first appeared for fundraising. This leela served as a test that ultimately deepened his faith. When he learned the truth, his doubt turned into a 'surge of love' and profound shame for having misjudged his Guru.