The baptismal character marks one as a Christian, while just being in the state of grace does not. All men, whether Christian or not, can repent of their sins (as the story of Jonah and Ninevah shows). All men, thus, can obtain grace before God. But the SACRAMENT of baptism alone, provides the indelible mark, which is distinct from the grace of the sacrament (justification).
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St Paul speaks of being "sealed" with Christ. He also says baptism makes one "clothed with Christ". It's not clear that BOD provides any of this, because the character of baptism is distinct from the grace. St Thomas says the baptismal character can never be lost, while sacramental grace can be, through sin.