MABEL CHILTERN. Well, I delight in your bad qualities. I wouldn't have you part with one of them. (1.131)
Mabel represents the accepting, indulgent wife, in contrast to Lady Chiltern's demanding and imposing one.
SIR ROBERT CHILTERN: It is not the perfect, but the imperfect, who have need of love. (2.311)
In a long, melodramatic speech, Sir Robert encourages Lady Chiltern to accept his past mistakes and the possibility of future ones.
SIR ROBERT CHILTERN: All sins, except a sin against itself, Love should forgive. (2.311)
What does Sir Robert mean here? Perhaps that only a loveless gesture – for example Lady Chiltern's emotional abandonment of her husband – can't be forgiven. But Sir Robert stands by his description of masculine love and happily forgives his wife.
LORD CAVERSHAM. Oh, damn sympathy. There is a great deal too much of that sort of thing going on nowadays. (3.66)
Lord Caversham stands by an old world, pull-yourself-up-by-your-bootstraps ethic that doesn't expect to give or receive help.
SIR ROBERT CHILTERN. She stands apart as good women do - pitiless in her perfection – cold and stern and without mercy. (3.153)
At this moment in the play, Sir Robert feels so alienated that his wife's "good" nature and "perfection" are enemies almost as formidable as Mrs. Cheveley's wickedness.
LORD GORING. She loves you, Robert. Why should she not forgive? (3.154)
For Lord Goring, forgiveness and love belong together. He's a good friend of Lady Chiltern's, but doesn't understand her notion of idealistic love. With his knowledge of his own flaws, maybe he can't imagine anyone loving him if he or she couldn't forgive him.
LORD CAVERSHAM. He is very heartless, very heartless. (4.66)
Lord Caversham repeats this phrase several times in the play. He sees only the public Lord Goring – lazy, indifferent, devil-may-care – not the real Lord Goring scrambling to save a marriage and advocating for a reliance on the heart.
LORD GORING. […] you whose lips desecrated the word love, you to whom the thing is a book closely sealed, went this afternoon to the house of one of the most noble and gentle women in the world to degrade her husband in her eyes, to try and kill her love for him […]That I cannot forgive you. (3.258)
It's interesting that Lord Goring can't offer Mrs. Cheveley what he so forcefully promotes to Lady Chiltern – forgiveness.
LORD GORING: Women are not meant to judge us, but to forgive us when we need forgiveness. Pardon, not punishment, is their mission. (4.236)
Lord Goring makes this speech after he's become engaged. How much of this is wishful thinking?
LADY CHILTERN. You can forget. Men easily forget. And I forgive. That is how women help the world. I see that now. (4.248)
As a modern reader, it's hard to get around the sexist message of the play's end. One other way of looking at it is this: formerly protective of Sir Robert's impermeable public persona, Lady Chiltern sounds like she'll redirect that energy into caring for the man himself, in all his humanity. So part of her transformation is abandoning idol-worship for the difficult, day-to-day work of accepting and loving a real person. And this interpretation is a little more palatable for someone reading An Ideal Husband today.