John Milton begins his Second Defence of the English People by stressing the universal importance of gratitude: "In the whole life and estate of man the first duty is to be grateful to God." Peter Medine has shown the prominence of gratitude in Paradise Lost, but scholars have not fully appreciated the role of this virtue elsewhere in Milton's writing. This thesis is an attempt to redress that oversight with reference to A Mask and Paradise Regained, while also answering a question that Medine raises but does not satisfactorily resolve: Why gratitude? Both texts have been read as responses to the early modern debate about the doctrine of things indifferent, or adiaphora, and I argue that this context helps explain Milton's interest in gratitude. The first section of this thesis accordingly reviews the historical and theological context of the adiaphora controversy, while the second examines Milton's more direct treatment of things indifferent and gratitude, primarily in De Doctrina Christiana. In the remaining sections, historical and literary analysis of A Mask and Paradise Regained illuminates how Milton addresses tensions in the doctrine of things indifferent by emphasizing gratitude. Of the commonly recognized criteria for directing the use of adiaphora—the rule of faith, the rule of charity, and the glorification of God, often through gratitude—gratitude toward God frequently receives less thorough attention, yet Milton gives it a prominent role in A Mask and allows it to overshadow the other guidelines in Paradise Regained. Although gratitude is itself sometimes subject to manipulation in these texts, both A Mask and Paradise Regained suggest that the requirement of God-ward gratitude can serve as a check against subtle distortions of the other guidelines. The effectiveness of this strategy stems from the fact that the vices gratitude guards against—self-indulgent ingratitude, stoical ingratitude, and idolatry—are the vices that underlie licentiousness and superstition, the primary abuses of the doctrine of things indifferent. Milton's privileging of gratitude thus provides a way of cross-checking appeals to the more contested criteria of faith and love, protecting the doctrine of things indifferent from perversions that would undermine Christian liberty.