My bad,
the correct Latin phrase (adapted into the English) is:
reductio ad absurdum
meaning 'reduction to absurdity'
definition:
Logic the proof of a proposition by showing
its opposite to be an obvious falsity or self-contradiciton,
or
the disproof of a proposition by showing its consequesnces to be
impossible or absurd when carried to the logical conclusion
Heb 6:4-5 (KJV1611 Edition):
For it is impossible for
those who were once inlightned,
and
haue tasted of the heauenly gift,
and
The earth and everything in it is passing away, but the man who does the will of God Abides forever. 
,
5 And
haue tasted the good word of God,
and
[have tasted] the powers of the world to come;
IMHO
SAVED =
those who were once inlightned,
and
haue tasted of the heauenly gift,
and
were made partakers of the holy Ghost,
5 And
haue tasted the good word of God,
and
[have tasted] the powers of the world to come;
So we can read the whole argument somewhat easier
if we say:
Heb 6:4-6
For it is impossible for those who were SAVED;
6
If they shall fall away,
to renue them againe vnto repentance:
seeing they crucifie to themselues the Sonne of God
afresh, and put him to an open shame.
Easier to see is that those who were saved,
should they fall away from grace,
then they can't be resaved.
BTW, the Doctrine of Security of the Believer doesn't
hinge on the 'if' starting verse 6.
John 3:16 (KJV1611 Edition):
For God so loued ye world, that he gaue his only begotten Sonne:
that whosoeuer beleeueth in him, should not perish,
but haue euerlasting life.
Everlasting life that stops (i.e. gettin unsaved) does not
qualify as 'everlasting life'.
Does anybody want to have a topic on:
What should I be doing now that i'm saved
to show i've been saved? GOOD WORKS would make
a better title, for it is good works that is what
we should be doing now that we have been saved
and can be SURE WE REALLY ARE SAVED.