We need to recognise that this quote, that comes from Act III scene 5, is not said seriously, and is part of a scene that comes as something of a comic interlude between Launcelot, Jessica and Lorenzo whilst the real action is about to take place in Venice. Launcelot, opening the scene, expresses his fear that, based on the contemporary belief that the sins of the father are handed down through the generations, Jessica is damned because of her father and his sins. However, this is said ironically, and not seriously, for note how Jessica responds to this taunt:
I shall be saved by my husband. He hath made me a Christian.
This of course indicates the kind of light-hearted, irreverent tone that dominates this scene that provides us with comic relief. Launcelot throughout the play plays the role of a kind of clown whose purpose is primarily to amuse and to make the audience laugh with his irreverence and his attitude to life. This is just one more example of this in the play.