Hebrews chapter 10 speaks of how we entered into the Era of the second (or new) Covenant. Verse 9 says, ".... He takes away the First that He may establish the Second." Then, in verse 10 we learn that because of the new Covenant "we are sanctified through the offering of the body of Jesus Christ." Does that apply to everybody, to all who believe that Jesus Christ was crucified and died that we may live?
First of all, we have to accept the offering of the body of Jesus Christ or else there is no sanctification.
Then in Exodus 31:13 (also Ezekiel 20:12) we find, "... my Sabbaths ye shall keep; for it is a sign between me and you throughout your generations; that ye may know that I am the Lord that does sanctify you."
Therefore, in order to be sanctified through the offering of the body of Jesus Christ we not only have to believe that He, the Son of God, offered up Himself for us to enter into eternal life, but we also need to conscientiously keep holy (Isa.58:13,14) the bible Sabbath (or seventh day Sabbath) which is a sign between the Lord and us, that He is the one who sanctifies us.
Why is it so important to be sure about our sanctification? Hebrews 10:14 gives the answer, "For by one offering He hath perfected forever them that are sanctified."
for the enquiring mind
Sanctification through Consecration
at this Link