Catholicism introduced the whole concept of purgatory, which specifically avoids the idea of eternal suffering. Water baptism is not a requirement for grace (read: salvation), although it is a requirement to be recognized as belonging to the Church.
someone who knowingly/culpably rejects baptism is not going to purgatory. as a non-catholic, i think you are misinterpreting here.
this is verbatim from the Catechism,
> Baptism is necessary for salvation for those to whom the Gospel has been proclaimed and who have had the possibility of asking for this sacrament
Sure, there are other routes - but not for someone who has culpably rejected a water baptism. Again, from the Catechism,
> Purgatory is the state of those who die in God’s friendship, assured of their eternal salvation, but who still have need of purification to enter into the happiness of heaven.
> He himself explicitly asserted the necessity of faith and Baptism, and thereby affirmed at the same time the necessity of the Church which men enter through Baptism as through a door. Hence they could not be saved who, knowing that the Catholic Church was founded as necessary by God through Christ, would refuse either to enter it or to remain in it.336
Purgatory is not meant for those who culpably refused to enter, absolutely not.