And I would strongly suggest that those that do backslide and garden their hearts to the degree where they are finally lost, are is very rare breed. God's grace is powerful... His love and passion for us infinite... His faithfulness constant and personal, so while it is possible for a man to be lost after having known the truth, I don't think it would be inevitable or easily accomplished. It has to be a conscious deliberate decision. That cannot be easy.
Is it hard to come to faith? Is it difficult to deliberately believe? Is it difficult to become a Christian?
If there is a way in to becoming a Christian, logically the way out is similar. If it is easy to become a Christian; it is just as easy to become an unbeliever. God expects a person to see that sin is rebellion against a Holy God, it is an expression of unbelief. Scripture is consistent in telling us that our final salvation is conditioned upon a present tense faith in Jesus Christ. That "faith" assumes that rebellion has stopped, that we have repented (turned away) from those things that separate us from God, and that God will change us and free us from those things that would separate us from Him. If we rebelle and sin, that is anti-faith. While the Bible is optimistic about the continuance of the Believer in the faith, the concept of falling from the faith being somehow "difficult," is wishful thinking.
"For if we go on sinning willfully after receiving the knowledge of the truth, there remains no longer remains a sacrifice for sins." It is not the falling into sin that is ultimately fatal, but the continuance therein. While the "wages of sin" is still death... It is not so much the initial stumbling and falling; but the danger of continuing unrepentant. If you continue, God says that there no longer remains a Sacrifice for sins for you. But if the stumble and fall, they get up, repent, confess their sin and move on in faith. "If we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness."