He shall purify himself with it
That is, with the ashes of the water of purification made of them: and this was to be done first
on the third day;
from the time of his touching the dead body. Aben Ezra intimates, that there is a secret or mystery in this and the following number seven; it may respect the third day of Christ's resurrection, who, as he shed his blood for the expiation and purification of sinners, so he rose again the third day for the justification of them:
and on the seventh day he shall be clean;
which may denote the perfect state, or sabbath of rest, which remains for the people of God, when all Christ's purified and justified ones shall be clear of all sin, and be the spirits of just men made perfect:
but if he purify not himself the third day, then the seventh day he
shall not be clean;
whoever is not cleansed from his sins by the blood of Christ, shed for the remission of them, and is not justified from them by him that rose from the dead the third day, will never be cleansed in the world to come, or in the eternal sabbath; but it will then be said, "let him that is filthy be filthy still", ( Revelation 22:11 ) .