Dear friend,
It’s more than okay to oppose atheistic humanism. Obviously, everything should be done as a means to attain union with God, so humanistic principles done in union with Catholic teaching, and for God are indeed
very good.
You are probably learning about the rise of secularism in Europe, about 1400-1600. Though humanism is a good thing in the sense that the Church benefitted from technology to travel and do missionary work, make beautiful paintings and sculptures, and increase literacy to read holy and pious books, there is always a negative side to this as well.
So you can oppose this if it’s meant to replace union with God.
Though your textbook may be created by a catholic book company, it may still have biases in favor of secularism and religious indifferentism. It might also have “BCE” and “CE” instead of “BC” and “AD”, which can indicate that it isn’t very catholic.
PS. I had AP world history at a public high school, and I can relate to you
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And here’s an article on humanism from the Catholic perspective:
catholic.com/encyclopedia/humanism