Salvation is a gift. Faithful endurance is your part. All is of His grace.

For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt.  

Hebrews 6:4-6

For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries. Anyone who has rejected Moses’ law dies without mercy on the testimony of two or three witnesses. Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he was sanctified a common thing, and insulted the Spirit of grace? For we know Him who said, “Vengeance is Mine, I will repay,” says the Lord. And again, “The LORD will judge His people.” It is a fearful thing to fall into the hands of the living God. 

Hebrews 10:26-31

It is precisely because the warning of these Scriptures are so terrifying that many would like to limit it to a particular historical context or ethnic group, namely the Jews (Hebrews) who had come to God by faith in Christ. But the Scriptures are timeless and they are for everyone. Certainly the LORD carefully chose what would be recorded for those who would enter into the New Covenant across the passage of time. As for every other letter recorded in the Scriptures, apart from it's immediate audience and recipients, I am convinced this was written for you and me. Compare these warnings to the Hebrews with a very similar warning giving to the non-Jewish Corinthian believers:

Moreover, brethren, I do not want you to be unaware that all our fathers were under the cloud, all passed through the sea, all were baptized into Moses in the cloud and in the sea, all ate the same spiritual food, and all drank the same spiritual drink. For they drank of that spiritual Rock that followed them, and that Rock was Christ. But with most of them God was not well pleased, for their bodies were scattered in the wilderness.
Now these things became our examples, to the intent that we should not lust after evil things as they also lusted. And do not become idolaters as were some of them. As it is written, “The people sat down to eat and drink, and rose up to play.” Nor let us commit sexual immorality, as some of them did, and in one day twenty-three thousand fell; nor let us tempt Christ, as some of them also tempted, and were destroyed by serpents; nor complain, as some of them also complained, and were destroyed by the destroyer. Now all these things happened to them as examples, and they were written for our admonition, upon whom the ends of the ages have come. 

1 Corinthians 10:1-11

In this warning to the gentile Corinthians, the Apostle Paul makes it clear that the lives of the Hebrews who were led out of Egypt by Moses was for an example to them and written for their admonition. These Hebrews in the Old Covenant record (Old Testament) came out of Egypt by God's hand and had experienced Christ before their rebellion, and finally their destruction. Only two men, Joshua and Caleb of the generation who had known the bondage of Egypt, entered the promised land of Canaan because they were steadfast in their faith and obedience to God.

If we were to apply the same argument that some use to disregard the warning given in the letter to the Hebrews, then we must apply the same reasoning here and disregard the warning given to the Corinthians, unless of course we are modern day Corinthians by birth. Continuing in this way, we would pretty much find that none of the Scriptures apply to us today and we would be totally deceived and lost.

Therefore:

• I am to cherish my salvation that was obtained by grace through faith in Jesus Christ [Eph 2:1-9]. 
• I am to work it out with fear and trembling [Php 2:12]. 
• I am to press on, that I may lay hold of that for which Christ Jesus has also laid hold of me [Php 3:12]. 

Because:

• To him who overcomes will He grant to eat of the tree of life which is in the midst of the paradise of God [Rev 2:7].
• Those who are with Christ and in His army are those who are called, chosen and faithful [Rev 17:14]
• This race of faith requires laying aside weights and sin, and running with endurance. [Heb 12:1]

This is a very condensed presentation that has come out of my own study through the Bible. I have concluded, on the evidence of many more Scriptures and examples than you find here that, Salvation is a gift. Faithful endurance is your part. That is not to say that God sits back and lets us exert our own strength. Not at all. We could never endure without Him. But we do have the responsibility to feed diligently on His word, to obey His word, to pursue and cultivate the relationship with Himself that He has called us into. We have been given His word and His Holy Spirit to empower, teach and help us in every way necessary until we see Him face to face. This is how we work out our salvation and this is how we stand in the true grace of God.