Hebrews 3:12-13 (ESV)
Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God.  But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin.
The author has brought to the memories of the Hebrew believers the story of the Israelites not being allowed into the promised land because of their unbelief. This was not to just give them something to think about. It was given as a warning to them. They fell into grievous sin because of their straying hearts (3:10). Now then, watch out that you do not have a sinful unbelieving heart that turns away from the living God, like they did.
God’s preservation of the elect does not cause believers to be careless with their hearts. It does just the opposite. The writer could have as easily said “prove the genuineness of your faith”. Real faith will prove its genuineness. It is not saying that we can lose our salvation to say that believers need to guard their hearts so that they do not turn away from God. Care for one’s spiritual health is a sign of true conversion and to use a biblical truth as reason to disobey a commandment of Scripture is not what a real believer will do.
Presumption is a sin that will end a lot of people in hell, not because they lost what they had but because they never had what they thought they had. Do you claim to be a follower of Jesus Christ? There have been others who made that claim who now will have nothing to do with the God of the Bible, who deny the truth of Scripture and who believe all kinds of non-Christian things. Don’t you be one of them. Examine your heart. Test your own faith. Show the reality of your faith through ruthless examination and rigorous sacrifice and cross bearing. The stakes are too high to do any less. Take the Scriptures seriously.