Calvin: why baptism?

i'm getting closer to finishing Calvin's Institutes. in the second-last chapter he has a go at the five sacraments rejected as unbiblical (confirmation, penance, extreme unction, orders and marriage). he doesn't have a problem with the first or last two, just with them being called sacraments in the same way as the lord's supper or baptism are.

in speaking against penance, he writes:

As if baptism were wiped out by sin, and is not rather to be recalled to the memory of the sinner whenever he thinks of forgiveness of sins, so that from it he may gather himself together, take courage, and confirm his faith that he will obtain the forgiveness of sins, which has been promised him in baptism!
IV.xix.17

that is, one of the key reasons for baptism is to recall, when you fall into sin, that you are one of those Christ died to redeem, and to go and sin no more.