[jo kus]

You're right. A seal that can be nullified isn't a seal, at all.

I disagree. Seals can be ignored by the recipient. Seals also can be broken, whether you admit it or not - for example, Pilate's seal on Christ's tomb. It still was a seal, despite it's limited power.

[annalex]

Well, unlike Pilate's lock, baptism is an indelible seal, but when a man sins, the seal of baptism is not broken, as the original sin is not regained through personal sin.