Brahman is the main purport of all scriptures

Topic 4: Samanvayadhikaranam [Samanvaya: coordination; adhikaranam: section, topic, proposition]

I.1.4
तत्तु समन्वयात्
Tattu Samanvayat
But that(I.1.3 Brahman is to be known only from the scriptures and not independently by other means is established), because it is the main purpose (of all scriptures)

Tat(तत्): that; Tu(तु): but; Samanvayat(समन्वयत्): coordination, on account of agreement or harmony, because it is the main purpose

Commentary:
Brahman or the absolute can be known only through the scriptures because all the passages of the scriptures can be harmonized only by such a doctrine. The scriptures refer to Brahman only because they have Brahman as their main topic. A very important point to be noted here is, Brahman cannot be realized through scriptures. Scriptures aims only at removing the ignorance about Brahman.

Background

Introduction - part1, part2, part3, part4

Sutra - I.1.1, I.1.2, I.1.3