MRS. CHEVELEY. Even you are not rich enough, Sir Robert, to buy back your past. No man is. (1.274)
The past seems like a wholly negative thing. A large part of Sir Robert's past is comprised of the positive decisions he's made. Now, only the mistakes seem important and influential.
SIR ROBERT CHILTERN. No one should be entirely judged by their past. (1.345)
Sir Robert says this about his nemesis, Mrs. Cheveley. He also seems to be fishing for a gentle response from his wife that might apply to him, too.
LADY CHILTERN. [Sadly.] One's past is what one is. It is the only way by which people should be judged. (1.346)
Sorry, Sir Robert. Lady Chiltern believes that people can't change. She refuses to believe that human beings are works in progress.
LADY CHILTERN. But you told me yesterday that you had received the report from the Commission, and that it entirely condemned the whole thing. (1.350)
Lady Chiltern has a memory for even the most recent past, holding Sir Robert to a decision he made yesterday.
SIR ROBERT CHILTERN. Gertrude, there is nothing in my past life that you might not know. (1.375)
Sir Robert wishes he could live in the truth. His fear of rejection makes that impossible at the moment. He lies.
SIR ROBERT CHILTERN. […] she looks like a woman with a past, doesn't she? (2.95)
What exactly does a woman with a past look like? Is it her dress? Her lipstick? Her age?
LORD GORING. Perhaps Mrs. Cheveley's past is merely a slightly DECOLLETE one, and they are excessively popular nowadays. (2.96)
Lord Goring reminds Sir Robert that Mrs. Cheveley might not be as susceptible to scandal as he is. As Lady Markby says elsewhere, it probably enhances her charms.
MRS. CHEVELEY. [With a sneer.] Oh, there is only one real tragedy in a woman's life. The fact that her past is always her lover, and her future invariably her husband. (3.253)
This quote is a brainteaser from Mrs. Cheveley. Does she mean that the past is romantic but erratic, the future steadfast but dull? That the best times are behind her? How would you interpret this quote?
SIR ROBERT CHILTERN. I wish I had seen that one sin of my youth burning to ashes. (4.170)
Sir Robert is referring to the old letter to Baron Arnheim. Would Mrs. Cheveley have been as powerful without this material piece of evidence? There's an interesting contrast between the past that truly existed and the past that's recorded – then later interpreted as reality.
LORD GORING. Why should you scourge him with rods for a sin done in his youth, before he knew you, before he knew himself? (4.236)
Lord Goring's understanding of human beings is fundamentally opposed to Lady Chiltern's. He believes a person becomes, she thinks a person is.