As the narrator will explain, this means that Dimmesdale's sin—sleeping with Hester, a married woman not his wife—had to do with being carried away by the emotion of the moment, nothing more. It had nothing to do with his principles being defective or with even a "purpose" or calculation in which he consciously decided he would sin. It was a brief lapse brought on by passion, and we learn that Dimmesdale, as a result, now monitors his "each breath of emotion, and his every thought."
At the end of the previous chapter, Hester, meeting with Dimmesdale in the woods, and seeing how oppressed he has been the last seven years by the weight of his sin, urges him to go away and start over somewhere new, perhaps by trying to convert the Native Americans. He, however, is too weak and broken to go alone. Hester says, no, don't go alone, and says she will flee with him.
Having once let his emotions carry him into sin, it is now easy for Dimmesdale, the narrator says, to repeat the same mistake:
But there is still the ruined wall [the initial sin], and near it the stealthy tread of the foe [Satan] that would win over again his unforgotten triumph.
Dimmesdale decides that he will flee with Hester. His emotions overtook him once and led him into sin. Now they are overtaking him again.