John Proctor has had an affair with Abigail. And he tries his best to keep this a secret. This is mostly to preserve his reputation. Elizabeth knows of the affair and has Abigail dismissed in order to save their marriage. Names play a central role in this play. Since the authorities and the courts illogically grant accusers immediate innocence while automatically assuming that those who are accused are automatically guilty, the act of naming becomes an act of conviction. To name someone, to accuse them, translates to convicting them. This is because Parris, Danforth, and Hathorne act thoughtlessly and impetuously.
So, when John is "named," everything about his life crumbles. His wife is also named. When John is in jail, Parris and Hale try to get him to confess to witchcraft in order to save his life. Proctor finally confesses but then refuses to sign a confession. He thinks it is enough that he has confessed verbally. He says that God knows his sins. (He is referring to the affair and not witchcraft of which he is innocent.) He asks why his confession must be made public. " Is there no good penitence but it be public?" Still refusing to tarnish his name, his legacy, and how his name will follow his children, Proctor cries out in anger why he refuses to sign:
Because it is my name! Because I cannot have another in my life! Because I lie and sign myself to lies! Because I am not worth the dust on the feet of them that hang! How may I live without my name? I have given you my soul; leave me my name! (Act 4)
John will confess his sins to God but he will not let his name be tarnished for his own sake, let alone in public support of the accusations of others.
Assuming you truly mean "topic sentence" instead of "thesis sentence," you could therefore state in you topic sentence that, because acknowledging a lie with the signature of his name will violate his good name and the legacy that good name leaves to his descendants, John is motivated in his actions by the preservation of his legacy.