Vedānta Deśika is the major systematiser of the Viśiṣṭādvaita Vedānta. Before him, Yāmunācārya and Rāmānuja set the theological bases of Śrī Vaiṣṇavism, making a theological and philosophical school out of it. Vedānta Deśika has to rethink its position in the landscape of Sanskrit philosophy. Since Sanskrit works start by rule with an indication of their epistemological legitimacy, Vedānta Deśika had to face at first the problem of the legitimacy of the epistemological background of Viśiṣṭādvaita Vedānta and Śrī Vaiṣṇavism. This included the Vedas but most of all a collection of Sacred Texts called ''Pañcarātra" and which was commonly used by Śrī Vaiṣṇavas in their religious praxis.
From this point of view, Vedānta Deśika's situation parallels the one of other Sanskrit philosophers, such as Bhaṭṭa Jayanta and Abhinavagupta, who had to face a similar issue in trying to justify the Śaiva Sacred Texts. Basically, Sanskrit philosophy knows to way to justify the epistemological validity of an instance of Linguistic Communication (e.g., a Sacred Text). Either it is said to be apauruṣeya, 'independent of a human [author]', and, hence untouched by his/her defects, or it is guaranteed by an authoritative author, an āpta, 'reliable'. The Mīmāṃsā school strongly advocates the first view, whereas the Nyāya one the second. Within the second, it is quite easy to accommodate one's belief in God as the author of Sacred Texts. He is their reliable author, insofar as he is the most reliable speaker altogether. And in fact, the Nyāya school itself soon enough identifies the ''reliable speaker" of the Vedas as God himself. The authors who follow this attitude, like Bhaṭṭa Jayanta, will just have to prove that a certain Sacred Text has also been authored by God to include it within the Canon of the valid texts. By contrast, Vedānta Deśika chooses to adhere to the Mīmāṃsā paradigm, and thus has to face a far more complex issue, i.e., justifying the validity of the Veda as independent of a divine author, while at the same time preserving the supreme position of God. Further, he needs to justify the authority of the Pañcarātra, although these are not apauruṣeya. This leads him to an articulate epistemology of Linguistic Communication.
What happens to a religion when it faces the challenge of istitutionalisation? What happens when it tries to found the validity of its Sacred Texts within an already established framework?
On Vedānta Deśika, you can see this post.