Well, of course the climax is the very top of the action in the plot of a novel. It is always found at the end of the rising action (where the suspense climbs). Therefore, in The Last of the Just, the climax is when Golda and Ernie are traveling on the final death train ride.
Ernie, the main character in The Last of the Just, has himself admitted to a Nazi death camp in order to be with the love of his life, Golda. He is savagely beaten and mistreated before gaining entrance because the Nazi soldiers suspected him as a spy.
The story comes to a climax as Ernie and Golda are in the train with many Jewish children who are all traveling on this final death train. Ernie is able at last to prove his worth and importance as he spends the time in the train explaining to the children how they are bound for a wonderful, blissful time and will soon be with their parents. He continues to console the children, even in the gas chamber, encouraging them to breathe deeply and hurry their passage.
The tone is imperative here. There is no better quote to show the tone of despair and gloom than this one:
And then, mother, the hair stands up on his head, for only a few feet from him in the darkness the enemy voice is reciting in Hebrew the prayer of the dying. Ai, God, the soldier has cut down a Jewish brother! Ai, misery! He drops his rifle and runs into no man's land, insane with shame and grief. Insane, you understand? The enemy fires at him, his comrades shout at him to come back. But he refuses; he stays in no man's land and dies. Ai, misery, ai...!
So, in addition to the climax, we have the continuing despair contained throughout the novel.