For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all. For he that said, Do not commit adultery, said also, Do not kill. Now if thou commit no adultery, yet if thou kill, thou art become a transgressor of the law. So speak ye, and so do, as they that shall be judged by the law of liberty. For he shall have judgment without mercy, that hath shewed no mercy; and mercy rejoiceth against judgment.
God gave the Israelites the Law, but He knew they could not keep it so He sent His Son to save us. He showed us great mercy in that one act. Instead of judging those of us who have accepted Christ's spilled blood for our sins, He will show us mercy.
There is no way we can keep the law. If we fail in just one aspect, we have failed in them all. We have all lied at least once in our lives (more like once a day!). We may not have murdered anyone, but we are just as guilty as someone who has. But praise God, He shows mercy!
We need to also show mercy to others as we have received mercy. Showing mercy is the ultimate form of loving our neighbors as ourselves which is the new law Christ gave before he died for us.