Where are you getting the "seals the righteousness of faith" language?
That's Rom.4:11 language.
The important thing is that one may not divorce the seal from the faith, or the faith from the seal.
If the observable seal is present, yet faith is not; yet faith may arrive provided the recipient is elect. The sacrament is
effective only unto faith.
We typically divide our use of the terms "seal" and "sign" as a convenient way of describing what baptism IS as God sees (seal) and as man sees (sign). Does God need a sign to point him in the right direction? No, so the sign is primarily for us.
God sees his seal perfectly, as he knows exactly when his sacrament of baptism is applied to someone who is, in fact, an elect child of God. Yet, he allows his sacrament will inevitably be applied to some who are not elect. He is still making the outward declaration, through his imperfect church administration, "this one is mine." And he is, to all external observation.
So, man sees baptism is also a seal, God making a claim on his creature as one redeemed, being incorporate into the visible church. Baptism isn't MY seal, it's God's. I recognize it (or claim to) as such when I see or experience it. For me, it points as a sign to realities, not only the ownership and citizenship claim of heaven, but also to the dying and rising of Christ, and all the rest of the biblical symbolism God has invested in baptism.
But it only serves well when it is united with faith. It points to the truth at all times, but only faith takes advantage of it. And, we don't need to worry when God's seal is falsified by human events, that is when a baptized person proves a reprobate. God was never fooled, either by a false profession or by the church's "misapplication"--which was not really such, since the administration always took into account the fact of apostasy.
Insofar as God chooses to speak through his imperfect church, he allows that men could mistake him. But the reality of the church applying the visible seal (as wax to an official document) helps demonstrate the horror of apostasy, of trampling underfoot the Son of God and despising the blood of the covenant, Heb.10:29. The "enlightenment" of Heb.6:4 I interpret as baptism, v6 warning of the danger of being ashamed of the Christ one once claimed. It is a real-and-true spitting on the seal of baptism, an actual offense at that claim God made on them.
Since men are not infallible, God ordained baptism to err on the side of inclusion, not exclusion. The paradigm case is Ishmael, followed by Esau. Because we don't believe (as paedobaptists) that the OT sign of circumcision was anything baptism is not, or that baptism is anything circumcision was not, we accept that God made the proper application of the sign "generous" in this sense. Man cannot know the outcome of election pertaining to any individual. So the church baptizes on precept, leaving the "margin of error" for God to determine.
Furthermore, as Presbyterians, we say a lawfully applied baptism IS a baptism, irrespective of when or even if a man has, acquires, or never exhibits signs of faith. That is because we don't rest the validity of baptism's signification or sealing on the baptized as the primary claimant (but as one who answers); rather on the God who claims sinners for himself.
