What convinces tom that what the devil has told him in the swap is true?

Tom is convinced of the devil's truth when he experiences a series of unsettling events that align with the devil's predictions, such as the sudden death of his wife. The tangible consequences of his actions and the eerie accuracy of the devil's promises lead him to believe that he is indeed in the grips of a supernatural force. Additionally, Tom's growing desperation and greed further cloud his judgment, making him more susceptible to the devil's manipulations. Ultimately, his fear and the reality of his circumstances reinforce his belief in the devil's claims.