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多角的な視点を持たねば、真実には至れないと、もはや自分は知っているのだから。

I don't have the slightest idea what たねば stands for. Is that another way of saying 持たなければ?

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「ねば」 is a literary way of saying 「もし・・ないなら」; therefore, it is used to form a negative if-clause.

"Because I already know that if I don't have diversified perspectives, I won't be able to reach the Truth."

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I don't know the reason, but since the two comments posted by me to OP's question were erased, I'll post nearly the same content as the comments as an answer.

Is that another way of saying 持たなければ?

Yes.

ねば means 「もし・・ないなら」 as is written in l'électeur's answer (+1 upvote).

The most important information about ねば in 持たねば is that it is an old expression of Japanese language for なければ in 持たなければ.

This expression is still relatively commonly used even now, as well as in written language, but sometimes used in spoken language.
However, since it comes from an old expression, there is a feeling/nuance of masculine, old fashioned, dignified and/or arrogant.

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