John Donne's Holy Sonnets present religious belief as an unsettled inner experience rather than as a simple statement of doctrinal certainty. The poems repeatedly move between faith, fear, repentance, self-accusation, hope, and the possibility of spiritual failure. This paper examines religious doubt in selected Holy Sonnets and argues that doubt in Donne should not be understood only as a rejection or absence of faith. It functions as a devotional form of questioning through which the speaker tests his relationship with God. The study uses close textual reading supported by historical and theological criticism of Donne's religious poetry. Particular attention is given to the poems “As due by many titles,” “Oh my black soul,” “This is my play's last scene,” “What if this present were the world's last night?” and “Batter my heart, three-person'd God.” The analysis considers the language of fear, legal obligation, bodily weakness, death, judgement, reason, and divine intervention. Earlier scholars have interpreted the sonnets through Calvinist despair, conversion theology, Protestant poetics, repentance, and the bodily experience of belief. The present study brings these approaches together but argues that the poems resist a single theological resolution. Donne's speaker doubts his own spiritual condition more often than he doubts the existence of God. His uncertainty therefore becomes a mode of dependence. The sonnets transform religious anxiety into prayer, and prayer into a continuing search for assurance. The paper concludes that Donne's religious doubt is productive because it keeps the speaker engaged with God even when certainty remains beyond his reach.