Lemme get a bit biology on you. Semipermeable membranes are a crucial part of the cell world. Surrounding certain cells, these membranes allow some molecules in, while keeping others out. If you're a human, they're crucial to your physiology. In short, you can't live without them.
Hold on to that thought.
In and of ourselves, we're pretty much toast before God. We can't stand up. One act of wrong doing and we're out (I know that seems awful picky of Him, but it's just that light and darkness can't be together). God acts rightly when He punishes evil. God acts rightly when He showers blessing on the righteous.
That's where we need that membrane.
In Jesus, everything changes. All the good that Jesus deserves, we get, because He lived perfectly in our stead. All the wrath we deserve is blocked, because we're seen through Jesus who did no wrong, who took on all of our punishment. So, our peace comes not because God decided to turn a blind eye to the wrong-doing of humans. We're good because God gave us the ultimate solution in Christ Jesus, our semipermeable membrane.