"Because Israel's immorality mattered so little to her, she defiled the land and committed adultery with stone and wood. In spite of all this, her unfaithful sister Judah did not return to me with all her heart, but only in pretense, declares the Lord." NIV translation
Israel lost sight of God's holiness and the land was invaded with them going into exile. Judah saw all of this happen and yet did not really repent of their idolatry and turn back to God. During this time (the reign of King Josiah), there was a great revival and an apparent turning back to God but, as we see here, it was not based on the holiness of God and was not a true change of heart. The same thing happened many years ago in the USA and is happening even more today. Many appear to come to Christ but not with a recognition of God's holiness and our sin. The "message of love" has been preached without mentioning the problem of sin but you cannot understand God's grace until you understand sin and its consequences. Without grace, it is impossible to be saved.
"The Lord said to me, 'Faithless Israel is more righteous than unfaithful Judah.'" NIV translation
At first glance, this verse may seem hard to understand but Judah had the opportunity to see how God judged Israel and yet they did not repent. This is similar to what is going to happen during the great tribulation as described in Revelation. The people that are here on earth, at that time, will have the opportunity to see God's judgment in action and yet most will still not repent. (see Revelation 6)
"'Go, proclaim this message toward the north: "Return, faithless Israel," declares the Lord, "I will frown on you no longer, for I am merciful," declares the Lord, "I will not be angry forever."'" NIV translation
God sent the prophet to declare to the northern kingdom that there was still hope for them because of His mercy. That mercy is available to us as well no matter what we have done in the past.
"'Only acknowledge your guilt - you have rebelled against the Lord your God, you have scattered your favors to foreign gods under every spreading tree, and have not obeyed me,' declares the Lord." NIV translation
This verse refers to the fact that, in order to have a right relationship with God, we must acknowledge the fact that we have sinned. Once we admit that we are sinners in need of salvation, then we can accept the grace of God's forgiveness.