Stalin did not trust Hitler, rather he misjudged Hitler's strategy and intentions. The general consensus among the Soviet leadership was that Germany would not want to be involved in a two-front war.

While war with Germany was probably inevitable at some point, the Germans would want to wait until they had finished off Britain. Stalin desperately hoped to forestall the war with Germany and made many efforts to placate Hitler and show that the Soviets were still invested in the non-aggression pact.

Even when the evidence began to mount that the Germans were planning an attack, there was always an alternative explanation that Stalin could cling to -- perhaps these reports are lies being spread by British intelligence to undermine Soviet neutrality, for example.

Nevertheless, Stalin did move some forces into a defensive posture on the eve of the German attack, but was reluctant to launch a full-scale mobilization because he believed that mobilization would invite the very war that he hoped to forestall.