I think is what the author is trying to say is that it is better that there is a God who cares than one who does not care at all.
The word "unjust" means that sometimes things happen that cannot be justified. Bad things happen to good people. We do not know why God has chosen to take someone away from us or let bad things happen to them. To us, it is very unjust.
Indifferent simply means that it does not make a difference either way. It implies that God doe snot care. Even if we cannot understand why God has chosen something, he has done it for some reason.
God offers serenity to many people. Faith is also a very important concept here. People who believe in God have faith and they trust that whatever decisions God has made, they are just.