Why - if Jesus Christ is God's Son - did God let him die on the cross?

Because God intended this from the beginning. In fact, all this was recorded in the writings of the prophets long before. God had shown them that the Christ would die because, through the blood of Christ, he wanted to make a new covenant between himself and mankind. And when he died Christ bore the judgment which the world deserved. He suffered the Torments of the Grave so that we would not have to suffer them. He himself said:

“This is the truth. It’s the truth I’m going to tell you.

If a grain of wheat does not fall into the ground and die,

it will remain a single grain.

But if it dies, it will produce a great harvest”

(John 12:24).

He did not remain a single one, for he rose on the third day, just as he’d told his disciples he would. Then God lifted him up to heaven where he will be until the end of the world. Then he’ll come back to gather together all who believe in him and then he will take them with him into eternal life. And they are his great harvest.