This is one of the first Economic Policies designed to boost a specific yield ( Faith in this case) other than Gold or Production. It works by doubling adjacency bonuses of a District; this means that first, you have to have such districts, and second, they have to have adjacency bonuses if you are to make use of Scripture. What's more, districts which have higher native adjacency bonuses will benefit more from this Policy than ones with low bonuses.
Policies such as Scripture, whose value is based on adjacency bonuses, are more efficient in the early and middle game than those based on buildings within districts, simply because in the early game you haven't unlocked and/or built these buildings, while adjacency bonuses work from the very moment you establish the District. Later in the game, however, the latter type of Policies will give you a higher output than the former.
Civilopedia entry Edit
The oldest religious text found so far dates to ancient Sumer around 2600 BC. It was the first example of scripture, a text that is considered to be sacred or central to a religious tradition. The oldest of a still existing religion is likely the Rigveda of Hinduism, composed between 1700 and 1100 BC. The claim of God’s (whoever it may be) Word “made manifest” is an undeniable selling point to would-be converts; it is not surprising that the five largest existing religions all have a primary scripture: Buddhism (the Tripitaka), Hinduism (Vedas and Upanishads), Islam (Qur'an), Judaism (Tanakh), and Christianity (Bible).