Before you get too comfy in that easy chair, let's take a look at why the Old Testament was even written down. We are told in Paul's first letter to the Corinthians both why it was written and why we should pay close attention to the examples therein.
1 Cor 10:1-13
I want you to know, brothers, that our fathers were all under the cloud, and all passed through the sea, and all were baptized into Moses in the cloud and in the sea, and all ate the same spiritual food, and all drank the same spiritual drink. For they drank from the spiritual Rock that followed them, and the Rock was Christ. Nevertheless, with most of them God was not pleased, for they were overthrown in the wilderness.
Now these things took place as examples for us, that we might not desire evil as they did. Do not be idolaters as some of them were; as it is written, 'The people sat down to eat and drink and rose up to play.' We must not indulge in sexual immorality as some of them did, and twenty-three thousand fell in a single day. We must not put Christ to the test, as some of them did and were destroyed by serpents, nor grumble, as some of them did and were destroyed by the Destroyer. Now these things happened to them as an example, but they were written down for our instruction, on whom the end of the ages has come. Therefore let anyone who thinks that he stands take heed lest he fall. No temptation has overtaken you that is not common to man. God is faithful, and he will not let you be tempted beyond your ability, but with the temptation he will also provide the way of escape, that you may be able to endure it.
Not once, but twice we are told that the events of the Old Testament are examples for us and were written down for our instruction. God has not changed. The same God who struck down the pagan, wicked nations dwelling in the Promised Land is the same mighty God who sits on the throne today.
"Okay," you say. "Fine. He's the same God. But He deals totally differently with people now."
Yes and no. Granted, disciples of Jesus do not carry out the will and judgment of God in the way that Israel was instructed and empowered to in the days of its theocracy. On the contrary, disciples are instructed to "shake the dust off" our feet and leave if people will not listen to our patient attempts to tell them about the Good News of reconciliation with God (Matt 10:14).
But God still demands holiness and wholehearted obedience from His people. It is good to examine these things honestly in the Word of God, even if they are kind of scary, because the fear of the Lord is the beginning of wisdom. We are not to neglect so great a salvation as has been bestowed on us by God's grace. It is not to be taken lightly, and we are not to act as though we do not need to examine ourselves.
Look at the New Testament example of Ananias and Sapphira. I see their example as quite similar to Achan's.
But a man named Ananias, with his wife Sapphira, sold a piece of property, and with his wife's knowledge he kept back for himself some of the proceeds and brought only a part of it and laid it at the apostles' feet. But Peter said, "Ananias, why has Satan filled your heart to lie to the Holy Spirit and to keep back for yourself part of the proceeds of the land? While it remained unsold, did it not remain your own? And after it was sold, was it not at your disposal? Why is it that you have contrived this deed in your heart? You have not lied to men but to God." When Ananias heard these words, he fell down and breathed his last. And great fear came upon all who heard of it. The young men rose and wrapped him up and carried him out and buried him.
After an interval of about three hours his wife came in, not knowing what had happened. And Peter said to her, "Tell me whether you sold the land for so much." And she said, "Yes, for so much." But Peter said to her, "How is it that you have agreed together to test the Spirit of the Lord? Behold, the feet of those who have buried your husband are at the door, and they will carry you out." Immediately she fell down at his feet and breathed her last. When the young men came in they found her dead, and they carried her out and buried her beside her husband. And great fear came upon the whole church and upon all who heard of these things.
God instructs us through Paul, who quotes Deuteronomy in the epistle of 1 Corinthians.
1 Cor 5:12-14
What business is it of mine to judge those outside the church? Are you not to judge those inside? God will judge those outside. 'Expel the wicked man from among you.'
Just as Achan had to be purged from Israel so that Israel could conquer Ai and the rest of the Promised Land, so those who will not follow Jesus and persist in rebellion are to be expelled from fellowship (obviously, after following all Biblical instructions for such a thing, which is not the subject of this post).
In Hebrews, there are three times the Old Covenant is contrasted with the New. I'll tell you right now, the message given is not "relax! We're under grace!"
For if the message spoken by angels [the Old Covenant] was binding, and every violation and disobedience received its just punishment, how shall we escape if we ignore such a great salvation?
If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, but only a fearful expectation of judgment and of raging fire that will consume the enemies of God. Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. How much more severely do you think a man deserves to be punished who has trampled the Son of God under foot, who has treated as an unholy thing the blood of the covenant that sanctified him, and who has insulted the Spirit of grace? For we know him who said, "It is mine to avenge; I will repay," and again, "The Lord will judge his people." It is a dreadful thing to fall into the hands of the living God.
See that you do not refuse him who is speaking. For if they did not escape when they refused him who warned them on earth, much less will we escape if we reject him who warns from heaven. At that time his voice shook the earth, but now he has promised, "Yet once more I will shake not only the earth but also the heavens." This phrase, "Yet once more," indicates the removal of things that are shaken--that is, things that have been made--in order that the things that cannot be shaken may remain. Therefore let us be grateful for receiving a kingdom that cannot be shaken, and thus let us offer to God acceptable worship, with reverence and awe, for our God is a consuming fire.
The fact is that we have greater light (knowledge and understanding of God's purpose and actions through time) than those under the Old Covenant. We have the great benefit of being able to see the hundreds of prophecies predicting Jesus' coming, the history of His life on earth and the prophecy of His Kingdom's fruition. The New Covenant is not "lighter," one in which we may do whatever we want because of grace. Perhaps this is part of the reason Jesus instructed us and anyone who would follow Him to count the cost before committing to His Kingdom. God does not change, and His holy nature did not and never will abide with sin in the camp.
There are things under the ban in your midst, O Israel. You cannot stand before your enemies until you have removed the things under the ban from your midst.
Let us examine ourselves and walk the walk of faith and holiness before the Lord, understanding His kindness and severity (Rom 11) now and forever.