To be shaken is an idiom for being afraid. Alternate translation: "no one would be frightened away from trusting in Christ" (See: [[rc://en/ta/man/jit/figs-idiom]])
Paul assumes that everyone knows that it was God who appointed them. This can be made explicit. Alternate translation: "God has appointed us" (See: [[rc://en/ta/man/jit/figs-explicit]])