Hebrews 6:1-3
But what is “the doctrine of baptisms”? Not as if there were many baptisms, but one only. Why then did he express it in the plural? Because he had said, “not laying again a foundation of repentance.” For if he again baptized them and catechised them afresh, and having been baptized at the beginning they were again taught what things ought to be done and what ought not, they would remain perpetually incorrigible.
“And of laying on of hands.” For thus did they receive the Spirit, “when Paul had laid his hands on them”, it is said.
“And of the resurrection of the dead.” For this is both effected in baptism, and is affirmed in the confession.
“And of eternal judgment.” But why does he say this? Because it was likely that, having already believed, they would either be shaken [from their faith], or would lead evil and slothful lives, he says, “be wakeful.”
It is not open to them to say, If we live slothfully we will be baptized again, we will be catechised again, we will again receive the Spirit; even if now we fall from the faith, we shall be able again by being baptized, to wash away our sins, and to attain to the same state as before. You are deceived (he says) in supposing these things.
Source: Homilies on the Epistle to the Hebrews (New Advent)