Reverend Parris is motivated by his need to maintain credibility and authority in the community. At first, he fears that his enemies will ruin him with the knowledge that there is witchcraft, and in his own home. However, Mr. Putnam suggests, "Let you take hold of it here. Wait for no one to charge you -- declare it yourself. You have discovered witchcraft --." He tells Parris that Parris can get in front of the accusations, so to speak, by claiming that he, himself, has rooted out the source of the evil. Further, Putnam says, "Let you strike out against the Devil, and the village will bless you for it!" Eventually, this line of reasoning seems to take hold of Parris, and he realizes the truth of what Putnam has suggested. If he can take credit for recognizing the evil, then he keeps his authority and position.
These concerns seem to preoccupy him throughout. When John Proctor brings Mary Warren to court in Act 3, Parris immediately tries to discredit Proctor so that his and Mary's testimony will not be believed. Her confession that the girls are lying, if believed, would mean that Parris has been wrong all along, and he would then lose credibility. Rather than let this happen, he accuses Proctor and Mary of lying.