The words "struggle" and "work very hard" mean basically the same thing. Paul uses them together to emphasize the intensity with which they serve God. (See: [[rc://en/ta/man/translate/figs-doublet]] and [[rc://en/ta/man/translate/figs-metaphor]])
Possible meanings are 1) "the God who is alive," as opposed to idols, who are not alive, or 2) "God, who acts," as opposed to idols that do nothing, or 3) "God, who gives life to all living beings."
The understood information can be stated clearly. Alternate translation: "but he is especially the Savior of those people who believe" (See: [[rc://en/ta/man/translate/figs-ellipsis]])