To die, to go from life to death and then to be resurrected from the grave would be a real change, mutability, would it not?
However, is it not Catholic doctrine and teaching that God is unchanging, immutable? As described in The Catholic Encyclopedia,
“These are some of the Scriptural texts which clearly teach Divine immutability or unchangeableness, and this attribute is likewise emphasized in church teaching, as by the Council of Nicaea against the Arians, who attributed mutability to the Logos (Denzinger, 54-old No. 18), and by the Vatican Council in its famous definition.”
And if “God died on the Cross and resurrected on the third day.” would not that leave the Father and Holy Spirit as a ‘biune’ instead of a ‘triune’ God?
Because our existence does not end with death.
Good question though, and it is important to recall this is true even in the case of souls going to Hell. Hence when Christ descended into Hell.