Hosea 8:11 has a repetitive bit of repetition in the 11th verse of Hosea 8. It's actually kind of funny when you first read it:
"Because Ephraim has multiplied altars for sinning,
they have become to him altars for sinning."
Well, yeah, duh. They built altars, they got altars. Nothing to see here, just kind of obvious, right?
Slow down a minute, and look a little deeper. How much work went into building an altar? The surface for the sacrifice took some work, like precisely cutting a large stone or doing some high-quality metalworking. But there was even more to it than that. Take a look at this picture from Beth Shan, where the Philistines hung the bodies of Saul and Jonathan.
The remains of the city are in the foreground. Notice that big hill in the background, the one that looks a little unusually tall and more than a little unnatural? That's because it is unnatural - it's an artificial, manmade hill. Tons of dirt and rock were hauled to this site by generations of people building it a few feet higher every year, until they had an artificial hill towering over their city - what you may know as a "high place".
Yes, that hill is where the people of Beth Shan would have had their altars, Asherah poles, and any other instruments of pagan worship. Similar high places were found in cities all throughout Israel. When they wanted to put an altar on a high place, they didn't just find the highest natural hill nearby. They continually built bigger hills to hold their new, fancy, forbidden altars.
So when Hosea says that Ephraim "multiplied altars for sinning", this is what he means. In pretty much every town of appreciable size, Israel spent years hauling in multitudes of tons of material (without modern earth-moving equipment!) to build these high places, grew grass and trees on them, made large, ornate altars of metal or stone, and hauled them to the top. And all of that was just the preparation for sinning!
Some sins were committed on a whim, of course. But these sins - these altars of sinning - required years of toil to lay the groundwork for sin. Many of these required years of scheming, transgenerational effort, incredible cooperation, and enormous expense - all to prepare a place where they could properly rebel against their Creator. These sins didn't just happen. They were planned and prepared for.
After all those preparations, all that time and effort and expense, what are the chances they would not indulge in the sin? They went through all that trouble to set the stage for sin, so of course they followed through, and the sin became for them an essential part of life.
Now track with me here. How are we doing the same thing? There are some sins that you just do - they're the instant reaction, the momentary lapse in judgment, the failure to do what's right, etc. And while those are plenty bad, there are sins that are so much more dangerous - the scheming sins. The ones you plot how you're going to do them, and how you'll hide it, and how far you're willing to push, and how you'll get away with it.
Addictions come to mind - hiding booze around the house, plotting how to get alone to watch porn, moving money around to hide it for gambling. Adultery is often like that as well - it's often said that nobody wakes up one morning and just decides to cheat; it's the culmination of a long process of increasingly bad decisions, dangerous thoughts, and nursing smaller sins to help them grow. Or think of plotting revenge, or scheming to defraud a client, or cheat on your taxes, or a host of others.
All sin is horrible. But these types of sin that require deliberate planning and focused effort - they're a whole 'nother level of deadly. If you have a pet sin like that, now is the time to put it to death. Get some friends to be like Hezekiah or Josiah for you. Don't just stop sacrificing there (for a while) - tear those high places down entirely.
29 minutes ago