according to M-W.com, SWORN is the pp of SWEAR and it has no any other meanings.
Relavent meanings of SWEAR in this context are as follows:
2 a : to assert as true or promise under oath <a sworn affidavit> <swore to uphold the Constitution> b : to assert or promise emphatically or earnestly <swore he'd study harder next time>
In our context, Charles V more likely promised under oath (2a), not just emphatically or earnestly, to be the defender of GOD. Therefore Li's translation of the word is accurate.
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