I would not explain it any better

Regards

Thank you for the correct answer. However, it would be foolish to agree with the statement. In the 21st century, people believe in astrology, numerology, and other pseudosciences much more than in God. It seems to them that the fate ordained by the stars can lead them to success or solve problems.

Why cant it be an accepted faith?

The established churches are, by definition, the accepted faiths.
We are told that astrology counters (goes against) the established churches. Something that counters the accepted faiths cannot be an accepted faith.

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Just as astrology was for centuries (i) _______ faith, countering the strength of established churches,

we need a word that explains or shows us the concept highlighted

This word counteracts the power of the establishment which was the church and its standards ideas at the time
B is the only viable option. read carefully the word of the sentence. the key to solve them is always before your eyes

regards

Just as astrology was for centuries (i) _______ faith, countering the strength of established churches, so today believing in astrology is an act of (ii) _______ the professional sciences.


The idiomatic expression "Just as....so" indicate parallelism or parallel ideas.

The phrase "countering the strength of established churches" suggests something subaltern, going against established norms, and therefore an underground fits perfectly.

Since the same idea is repeated after "so today", we need a phrase similar to "countering the strength" and so the choice is defiance against.