Commentary of Charles Spurgeon, Adam Clarke & Matthew Henry
Wherefore lift up the hands which hang down,
and the feeble knees; 
And make straight paths for your feet,
lest that which is lame be turned out of the way;
but let it rather be healed.
Follow peace with all men, and holiness,
without which no man shall see the Lord:
Looking diligently lest any man fail of the grace of God;
lest any root of bitterness springing up trouble you,
and thereby many be defiled;
Vs.12-15
Wherefore lift up the hands - The apostle refers to Isaiah 35:3.
The words are an address to persons almost worn out with sickness and
fatigue, whose hands hang down, whose knees shake, and who are totally
discouraged. These are exhorted to exert themselves, and take courage,
with the assurance that they shall infallibly conquer if they persevere.
Make straight paths for your feet - That
is, Take the straight path that is before you, do not go in crooked or
rough ways, where are stones, briers, and thorns, by which you will be
inevitably lamed.
Follow peace with all men - Cultivate, as far as you possibly can, a good understanding, both with Jews and Gentiles.....follow it through all places; trace it through all winding
circumstances; and have it with all men, if you can with a safe
conscience. 
And holiness - That state of continual sanctification, that life of purity and
detachment from the world and all its lusts, without which detachment
and sanctity no man shall see the Lord.
Looking diligently - Looking about being constantly on your guard.
Lest any man fail of the grace of God - Lest any person should come behind, or fall off from this state of salvation, viz. the Gospel system or
Christianity; for this is most evidently the meaning of the apostle.
Lest any root of bitterness springing up - A
root of bitterness signifies a poisonous plant.
-----The root of bitterness is here used
metaphorically for a bad man, or a man holding unsound doctrines, and
endeavoring to spread them in the Church.
Trouble you - This alludes to the
effects of poison taken into the body.
He enters a serious caveat against apostasy, v. 15. Here
you may observe,
First, The nature of apostasy: it is failing of the
grace of God; it is to become bankrupts in religion, for want of a good
foundation...
Secondly, The consequences of
apostasy: where persons fail of having the true grace of God, a root of
bitterness will spring up, corruption will prevail and break forth. A root of
bitterness, a bitter root, producing bitter fruits to themselves and others.
Bad example and false teaching have corrupted thousands, and are still making desolation in the world and in the Church.
Peace and holiness are
connected together; there can be no true peace without holiness.
