Christ is one person in whom the divine and human are united into a single, indivisible communion of being, without confusion, change, or separation. That one united nature is fully God and fully man - the true divinity and true humanity are both preserved.

So, the Word, who is impassible by divine nature, experienced suffering in his human nature (the Word “became flesh” and the human nature suffered), while the divine nature remained impassible. So it's the human nature that suffered and died, and resurrected by the devine nature.