Rational demonstration in favor of God`s existence was a fundamental theme in Indian theism — īçvara-vāda (teaching about Īçvara), original theistic metaphysics in classical Indian Philosophy. Traditionally, Nyāya together with Vaiċeşika and Yoga are regarded as the most theistic Indian systems that assert Īçvara — personal Divine Absolute with such attributes as perfect and supreme consciousness, compassion, etc. Udayana (X-XI cc.), an eminent Indian philosopher of the Nyāya school, performed a highly-sophisticated and elaborated defense of God`s existence, especially in his brilliant opus called “Nyāya-kusumāņjali” (NK), where he presented his famous eight proofs in the severe polemics with opponents (nirīçvara-vādins). By the end of XX century, translators and interpreters of NK started the process of actualization of Udayana`s argumentation in detail. Cosmological proofs in NK present Īçvara as Supreme Cause of the universe, its numerical structure and initial impulse. Udayana`s theism resembles metaphysical principles of ancient, medieval philosophy and contemporary Thomism. Besides comparative analysis, this article undertakes the questions of God`s transcendence and immanence, co-relation between reason and belief in Udayana`s works. Lately, scholars have stressed that Udayana has been deprived of sufficient attention though his heritage and original rational arguments for God`s existence can enrich world theism and theology.