If there were only one righteous man! One perfect one, God would spare the entire nation for that man’s sake. Jeremiah 5:1, describes God’s challenge to Jeremiah to find that one man. The Lord knows that he will fail in his quest. He knows the hearts of all man and even the 
religious leaders of Israel could not be trusted. Jeremiah 5:2 indicts them. It says, “Though they say, ‘As the LORD lives,’ yet they swear falsely.” Even those who talk a religious game and use religious sayings and talk loud about themselves are ultimate failures in God’s eyes. Isaiah is right in that all of man’s righteousness is as filthy rags! The understanding of this profound truth helps us understand Jesus.
Ryken explains this very well. He writes, “If only the good man Jeremiah was looking for could be found! If only one man could be found who is honest in all his dealings, and who seeks the truth. If only one man could be found to be righteous for the people’s sake. If only that one man could be found, a man of such perfect integrity that he could turn away the wrath of God.” Yet we all know that Jeremiah found no one! But as Ryken continues, “There is that man—Jesus Christ. What about him? Can he meet Jeremiah’s conditions? Jeremiah was told to look for a man ‘who deals honestly.’ Jesus Christ was straightforward in all his dealings. He gave an honest presentation of his deity, performing miracles to prove his divine power over creation. Jesus Christ dealt honestly with his disciples, not hiding from them the necessity of his own sufferings and death. Jesus Christ also dealt honestly with sinners, like the woman at the well (John 4), exposing their secrets and inviting them to trust in him. And Jesus Christ dealt honestly with his enemies, like the Pharisees, confronting the enmity in their hearts. There was nothing false or deceptive in anything Jesus said or did.”
But in verse 1, Jeremiah was also told to look for one man who “seeks the truth.” Ryken goes on, “Jesus Christ not only sought the truth—he is the Truth! At the beginning of his Gospel, John says Jesus Christ came into the world “full of grace and truth” (John 1:14). That is truth with a capital T, the Truth of God himself. Thus when the disciples wanted to know the way to eternal life, Jesus said, ‘I am the way and the truth and the life’ (14:6). The challenge God gave to Jeremiah was answered in Jesus Christ. He is the one man, the plain-dealer, the truth-seeker, the righteous man for whose sake God can forgive his people.”[1]
[1] Philip Graham Ryken, Jeremiah and Lamentations: From Sorrow to Hope, Preaching the Word (Wheaton, IL: Crossway Books, 2001), 93.