There are multiple warnings, including two very strong and extended warning passages in

 Hebrews 6:4-8

4   For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,

5   And have tasted the good word of God, and the powers of the world to

    come,

6   If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.

7   For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God:

8   But that which beareth thorns and briers is rejected, and is nigh unto cursing; whose end is to be burned.

and Hebrews 10:26-31

26  For if we sin willfully after that we have received the knowledge of truth, there remaineth no more sacrifice for sins,

27  But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.

28  He that despised Moses' law died without mercy under two or three witnesses:

29  Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace?

30  For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people.

31  It is a fearful thing to fall into the hands of the living God.

  

There are lesser warnings as well, such as Hebrews 4.  And there are the four heuristics (interpretative frameworks) used to interpret the warnings in Hebrews:

 

          i)          The "Arminian" view, that these verses warn of loss of eternal salvation;

            ii)         The hyper-Calvinist view, that they warn that one may "never really have been saved"

            iii)        The inheritance view.  If you defect from the Christian faith, you forfeit your inheritance, the privilege of ruling and reigning with Christ.

            iv)        The smorgasbord (or "variety pack") view, each of these warnings is directed at a different subject.  There is no specific theological direction of this Epistle.

 

The Message and the Warnings: Four Views