Security of Salvation

I have been debating the issue with myself :) about if Christians can loose their salvation. Here is a verse that is good proof against that, "This is the Father's will which hath sent me, that of all which he hath given me, I should loose nothing, but should raise it up at the last day." John 6:39 Passages that seem to say it may be lost are Hebrews 6:4 to 6 and 2 Peter 2:20 and 21. Another verse to consider for the once saved always saved stance is 1 Jn. 2:19.