What was the firm resolve made by the Deshmukhin, Radhabai, and why was she so insistent on receiving a mantra directly from Sai Baba?

Radhabai, the Deshmukhin, made a deeply firm resolve in her heart that she would not leave Shirdi or go elsewhere until Sai Baba personally gave her a secret mantra and made her a recipient of His grace. According to Chapter 18, she considered any instruction or mantra taken from another source to be "impure." Her conviction was that the foremost of saints, Shri Sai, must be the one to bestow this grace upon her. This profound belief in Baba's supreme spiritual authority drove her to undertake an extreme fasting protest, giving up all food and water to demonstrate her unwavering faith.