Grace unto you, and peace,
from God our Father and the Lord Jesus Christ.
The word of God gets to the heart of the matter in Hebrews 4:12 by firstly dividing asunder soul and spirit (Adam1 and Adam2 in 1Cor 15), then also dividing asunder joints and marrow; Which is allegory for firstly dividing Law1 vs Law2 in Romans 8, then dividing Laws vs Grace, to know both the thoughts and the intents of the heart. For a heart not established with grace only can defile itself with it's own mouth.
http://www.godshew.org/Hebrews5.htm
So let us also note that rightly dividing the word of truth in 2Timothy 2:15 not only rightly divides laws vs grace to grace, but furthermore to grace only in order to be approved(only) unto God, not both disapproved and approved:
http://www.godshew.org/RightlyDividing.html
So getting the heart "established with grace" in Hebrews 13 involves letting the law be "dead testator" of the New Testament in Hebrews 9. For in Matthew 12:30 it's not both with and against, but with "or" against; A matter of grace or law, not both, since both is an oxy-moron with a bad ending, such as "more child of hell(law)" in Matthew 23:15, or "worse" case scenario as the latter state of man in Matthew 12:45 (Luke 11;26) played out in the Revelation of John, who turned back to law behind him.
http://www.godshew.org/Allegory3.htm
http://www.godshew.org/Revelations9.htm
But thank God worst does not occur in the New Testament, and even a "worse" case scenario of the latter state of man ends with grace only, when we endure unto the end (of law) written to be saved only by grace only:
The grace of our Lord Jesus Christ with you all. Amen.