Hebrews 6:1-8 has been incorrectly used to support works righteousness, and legalism for years. This misunderstanding of scripture comes from eisegesis and poor hermeneutics. It, in fact, is an agreement for perseverance of the saints. We will look at just the section of scripture to begin. Then we will give it context within Hebrews and then within the entirety of the new covenant. I think it will be helpful to those who have been struggling with their own justification and soteriology.  Hebrews 6:1-8;
Therefore leaving the elementary teaching about the Christ, let us press on to maturity, not laying again a foundation of repentance from dead works and of faith toward God, of instruction about washings and laying on of hands, and the resurrection of the dead and eternal judgment. And this we will do, if God permits. For in the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, and then have fallen away, it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God and put Him to open shame. For ground that drinks the rain which often falls on it and brings forth vegetation useful to those for whose sake it is also tilled, receives a blessing from God; but if it yields thorns and thistles, it is worthless and close to being cursed, and it ends up being burned.
     Here is a link to a book introduction from Dr. John Macarthur that will give us the context I was mentioning earlier; Book introduction for Hebrews

     Many have been confused by this section of scripture. It has been used to argue against perseverance of the saints. Perseverance has been misunderstood by some as to mean eternal security. They claim that eternal security is a false teaching, and if wrongly understood, I would agree. We don’t believe that one will be saved, and remain saved, while practicing all sorts of ungodliness in unrepentance. It is as God said through Paul in Romans 6:1-2;
“…1What shall we say then? Are we to continue in sin so that grace may increase?2May it never be! How shall we who died to sin still live in it?…”
John 14:15; “…15“If you love Me, you will keep My commandments…” and James 2:14-17, “…14What use is it, my brethren, if someone says he has faith but he has no works? Can that faith save him?15If a brother or sister is without clothing and in need of daily food,16and one of you says to them, “Go in peace, be warmed and be filled,” and yet you do not give them what is necessary for their body, what use is that?17Even so faith, if it has no works, is dead, being by itself. …”
     Here is a link from Matt Slick of CARM.org about perseverance of the saints; http://carm.org/can-believer-lose-his-salvation
     Here is a great article from CARM.org also dealing with this issue; http://carm.org/about-can-person-lose-salvation
     And here is another from GotQuestions.org that will also help us to see the context in the new covenant; http://www.gotquestions.org/license-sin.html

     A person, who is truly justified by God, granted faith and repentance, who has been regenerated by God, simply does not choose to fall away, or become apostate. They did not save themselves. God did the saving to them. How many of you pray for person to be saved, when you know they want nothing of God, and claim to hate Him? You don’t seem to care about their alleged free will at that time do you? No, you want God to change their heart. So for all who truly believe, it is an act of the Sovereign God miraculously regenerating them to justification and on through the ordo salutis. If Christ did the work to justify us, then what work could we do to negate the perfection of His? None! In keeping with the spirit of salvation, we are changed, born again, a new creature with new desires, and affections. This new creature, loves the things of God, and hates the sin of his past existence, and bondage to his fallen nature. In this state of God granted repentance, and regeneration the new creature, of his new will and volition, derives his joy from doing that which is of the Father’s will, as our Lord and Savior Jesus Christ did upon the cross of His crucifixion. For this creature there is no ability in him to apostatize. Only for the religious person, who has not been freed from the old nature can fall away in any real sense, alas they were never really in Christ, so their falling away is even a false apostasy, thus the hyperbole of this scripture. For the one made to be resurrected unto eternal life, there are no sins of his account, to be tallied against him on the day of judgment. They had all been expiated, through the propitiation of Christ Jesus. Thus, for them there would be no grounds to repent, and believe again, or to crucify Christ, who once and for all died once, and was resurrected to never be crucified or die again. The absurdity of this concept should propagate the ideology of perseverance not the antithesis.
We fall into this heresy when we don’t understand justification and sanctification rightly.      When we think we had something to do with our justification we tend to believe we need to keep ourselves saved. This view is held by all works righteousness religions. The practitioner maintains their state of pseudo-grace by doing deeds and sacraments. Well, we all know this is a heresy, so why do we fall for it? Idolatry and pride, that’s why. We still want to earn it. We want to be better than others. Then there is guilt and pride. You are so bad, look at you, of course there isn’t enough grace for you, you must add something. These are powerful adversaries to fight against because they are stealthy. They sneak in to rob you of your security and joy. They take from you the delight you have in the Lord of your salvation. When these appear we need to crop off their ugly heads with repentance and prayerfully remember who it is that did the saving. It is the Almighty God, Lord of Lords, and Eternal Creator, who justifies you! Justification is solely the work of Jesus. He did the work on the cross of atoning for our sins and imputing His righteousness to us. That is how we are justified.
     Sanctification on the other hand involves our efforts in obedience. We try our best out of the provisions of God to affirm His will and be conformed to it. Notice we aren’t doing it out of our own provisions because we have none of any efficacy. Our pitiful temporary repentance and faithless intellectual understanding don’t do a thing for our sanctification. It is also the Lord who sanctifies us to Him through His provisions of all that is necessary. We through the true repentance of sins and the true faith are in concurrence with God and desire His will be done to us and by us.
     So the difference between the two doctrines should be clear. In justification God does the saving and keeping. In Sanctification we do the agreeing and growing. We are like a baby in a crib. We can think about growth and desire it. We can focus all of our efforts towards it, but we will grow according to His will. It is His design for a baby to grow to maturity. The immature believer will grow by design. Of course here is where the synergism comes in, like a child we must be fed good things and given work and exercise. We must be nurtured and disciplined. God being our Good Father sees to it. We are part of our sanctification for sure, but it is not what keeps us saved. I hope this has helped.