But that's not what 2 Pet 2:20-22 says about it:
"20 If they have escaped the corruption of the world by knowing our Lord and Savior Jesus Christ and are again entangled in it and are overcome, they are worse off at the end than they were at the beginning. 21 It would have been better for them not to have known the way of righteousness, than to have known it and then to turn their backs on the sacred command that was passed on to them. 22 Of them the proverbs are true: “A dog returns to its vomit,”[a] and, “A sow that is washed returns to her wallowing in the mud.”
(1) Those who have truly known Our Lord and Savior can again turn away from Him.
(2) When they do this, the latter state is worse with them than the beginning.
(3) This passage clearly shows apostasy from Christ is worse than not knowing Him.
As for Seal, that refers to an External/Visible Sign like Baptism/Circumcision etc.
Reference: "11 And he received circumcision as a sign, a seal of the righteousness that he had by faith while he was still uncircumcised. So then, he is the father of all who believe but have not been circumcised, in order that righteousness might be credited to them."
Once a person was circumcised, he could not be circumcised again, in the OT.
Likewise, in the NT, once a person is Baptized, he should not be baptized again.
If he becomes apostate, he doesn't lose the seal of Baptism. He loses the indwelling Grace of the Holy Spirit.
When after his apostasy he repents and comes back to Christ, he is not baptized again. But he can be forgiven and restored again, when he confesses his sin of apostasy etc, and the Lord washes it away, as it says in 1 Jn 1:9 also.
This is the sense in which Scripture speaks of
"One Baptism". One in the sense that Baptism is not re-done twice etc.
God Bless.