Firstly, what is salvation? It has to do with the acceptance of the message, sum and sunstance christianity is about. let us read this; Acts 4:12, Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved. John 3:16-17, For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. For God sent not his Son into the world to condemn the world; but that the world through him might be saved. Luke 19:10,For the Son of man is come to seek and to save that which was lost. Romans 6:23, For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.In a nutshell the above scriptural verses tells us what we need do and why Christ, our lord and saviour, came to this earth. when we receive this message and accept christ into our life, we are saved and have salvation.[Revelation 3:20, Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.]

Now,the question asked is can one lose his salvation, having been saved, in light of this scripture from the book of Hebrews 6:4? which reads, 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.Let me start off by saying that this passage has given room to a lot of interpretation and it behoolves us as teachers and apologists to always look at it in light of the prevailing background and the context of that verse. One way to understand and fully grasp a passage is to follow it from the opening verse or chapter. To look at this passage with the view to get to know if indeed, we can lose our salvation, we must go to the opening. In verse 1-3,we read, Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God, Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment. And this will we do, if God permit. It is obvious here, that it deals with a strong encourament togrow in the things of God. But why would the author write this? Let us remember, that after the ascention of Christ the persecution of the christians continued for then the church was birthed. It was this background that prompts the writer. The converted jews often incurs the wraith of the highpriest who were very influncial those days and can do a lot pf harm and suffering to the professed christians and thier family. Like kicking th chidren out of the synagogue school and other forms of persecutions in some cases, even imprisonment or death. This caused some not to be open about thier faith, attend gatherings or be seen in the mists of the bold ones. By taking this action they could not get to know more about Christ. Like paul says in the book of Philippians 3:10, That I may know him, and the power of his resurrection, and the fellowship of his sufferings, being made conformable unto his death; The passage is telling those in the new found faith to get to become intimate with Christ, pray more, and live the christian life without shame or feaer of the gospel. see Romans 1:16, For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek. The Arminian theology says indeed, on can lose his salvation referring to Hebrews6, but we know better. As christian we are to search the scripture everyday and do our reserh on a daily bases, paul says of the people of berea in Acts 17:11, These were more noble than those in Thessalonica, in that they received the word with all readiness of mind, and searched the scriptures daily, whether those things were so. No man is greater than the scripture, no man knows more than the scripture. Therefore, any interpretation that does not fall in-line with the scripture should never be taken seriously just like the people of Berea did and attracted the commendation from the apostle Paul.

Finally, let me say with scriptural backing, that we are secured in christ having been saved. John 10:28,And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. If a christian sins and remain in sin God will discipline him just as a father does his children see Hebrews 12:7-10 If ye endure chastening, God dealeth with you as with sons; for what son is he whom the father chasteneth not? But if ye be without chastisement, whereof all are partakers, then are ye bastards, and not sons. Furthermore we have had fathers of our flesh which corrected us, and we gave them reverence: shall we not much rather be in subjection unto the Father of spirits, and live? For they verily for a few days chastened us after their own pleasure; but he for our profit, that we might be partakers of his holiness.
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Since when have you people not lost it?

Matthew 24:13
[13]But he that shall endure unto the end, the same shall be saved.

If you leave the faith, you will loose it wotowoto.

Open your eyes and read that hebrew verses you quoted again.