Dimmesdale's sin is also fornication, because he has sex outside of marriage. In addition, he is a hypocrite and a coward for not admitting he is Pearl's father and letting Hester suffer alone when he should have been right there by her side. Further, Dimmesdale is a preacher. In the Bible, religious leaders are held to a higher standard than the rest of the faithful because people look up to them for spiritual guidance. This makes Dimmesdale's conduct even more reprehensible. God takes a dim view (no pun intended) of those men and women that are supposed to be spiritual leaders and have lives that are controlled not by the spirit but by sin. It isn't that people don't sin, we are all sinners, but we repent of our sin and ask forgiveness. Dimmesdale does not do this, instead trying to hide his sin. But as the novel unfolds, we see how this hiding of sin winds up destroying Dimmesdale.
In my opinion, Dimmesdale is guilty of two sins, the sin of pride and the sin of false witness (in addition to the sin of adultery).
I say this because Dimmesdale does not admit to the fact that he is Pearl's father and the man with whom Hester had the adulterous relationship. When he does not admit to it, he is, in a way, bearing false witness.
He does this because he wants to keep his position as minister. To me, that shows that he is proud -- he wants to maintain his position in the society even because it means something to him to have that position.
In the book "The Scarlet Letter" the Reverend Dimmasdale also would have been considered as being guilty of committing adultery.Even though he is not married, he did have relations with a married woman. As time goes by the Revered is able to build his congregation and save some souls but his own soul is always tormented by his act. He knows that he is also guilty of living with a lie before God and the congregation. It is his inability to live with his lie that has resulted in him having the letter impressed upon his own chest and his reason for revealing the truth before he dies.