In order to sigh and cry successfully, we must believe God. This is vital! In this context, it means that we need to believe how He defines sin. For instance, we must never come to think that "weeping for Tammuz" (Ezekiel 8:13-14) is really not all that bad. God calls it an abomination! If He calls it that, that is exactly what it is, and we need to accept His definition.
To use a more contemporary example, many "good" folk in the world observe Christmas, sincerely believing that they are worshipping God. They will actually say, "This is how I worship God," but we understand that how they worship God does not amount to a hill of beans! Only God can define how we are to worship Him, and it is for us to follow Him and act accordingly. We need, then, not just to know the law, but also to believe that it defines sin for all time.
Some people can see sin right before their eyes, they can hear it around them, they can live amidst it, but they can never sigh and cry over it because they refuse to allow God's law to be the standard of their behavior. History is replete with examples of this, but we will look only at one. Who of the Jewish leadership—except for Joseph of Arimathea, Nicodemus, and perhaps a few others—sighed and cried over the perpetration of an illegal trial that resulted in Christ's death?
In John 16:20, where He is speaking to His disciples on the evening before His crucifixion, Jesus says, "Most assuredly, I say to you that you will weep and lament, but the world will rejoice." The world, rejecting God's standards, rejoices at injustice and sin. Human nature can and does rationalize large-scale sin and social injustice, such as the Holocaust, sin that fills the land with vast violence. It can simply rationalize such atrocities on racial, economic, and religious grounds.
We in God's church must come to avoid partiality, mentioned in Leviticus 19:15, as we interpret the news and the social injustices that we see around us. After all, God did not ask Ezekiel to identify Israel's sins in his tour of Jerusalem in Ezekiel 8. God identified the sin for him, even when it was committed in secret. God calls out the sins in His Word, defining the abominations in His law, and we need to know those laws and believe that they are indeed sin. And we need to cry and sigh.