What is the surest way to rationally found one's believes?

Vedānta Deśika (XIII c.) was a polygraph. He wrote in different genres and in three different languages. However, the Seśvaramīmāṃsā (SM) occupies a specific position in his production, insofar as in it Vedānta Deśika explicitly faces the orthodox tradition of Mīmāṃsā. Thus, it represents at the same time Vedānta Deśika's essay of making Śrī Vaiṣṇavism compatible with the Vedic orthodoxy and of showing how Vedic orthodoxy would be useful and welcome for Śrī Vaiṣṇavas.

In his SM, Vedānta Deśika mainly focuses on orthodoxy, whereas problems concerning the Śrī Vaiṣṇavas' orthopraxy and the legitimacy of their rituals are dealt with in other works (see Rastelli 2006). Similarly, the emotional answer to the question about God's existence is dealt with in the copious devotional poems composed by Vedānta Deśika. Consequently, the SM represents an intellectual enterprise, one aiming at creating a synthesis between the two systems. How far can this synthesis reach? A main obstacle seems to be the Mīmāṃsā atheism, which would frontally oppose the very foundation of Śrī Vaiṣṇavism.
Vedānta Deśika will have to detect a difficult path within his interpretation of the foundational texts of Mīmāṃsā (Jaimini's Mīmāṃsāsūtra and Śabara's Śābarabhāṣya), one which allows him to say that the atheism as conceived by Mīmāṃsā authors was not a denial of the god devotees worship and at the same time to ground Śrī Vaiṣṇavism through rational argumentation independent of the pre-postulation of God's existence.

I have been blogging a lot about Vedānta Deśika, see for instance this post (on the epistemology of direct perception), this one (on that of dharma), this one (on linguistic communication), this one (on intellectual intuition).