Read carefully what the written word of God says, and then compare it to what your false teachers said about it...
See here where your teachers talked you into thinking that humans are only evil...that is not what Paul is explaining.
Paul is quoting the Old Testament scriptures, and he is explaining that the Jews sinned too, not just the Gentiles.
Paul is quoting about a specific time in Israel when the people were especially bad and did not seek God.
Paul is NOT speaking about all the people all the time.
What is King David saying where Paul quotes?
Listen carefully; Paul is quoting king David who at that time in which David speaks, the people are in a bad condition inside Israel. When the day spoken of arrived, the day where king David says what he says---no one searched for God.
However, If no one ever searched for God, ever, then how was it that the Israelites called out to God to help them when they thought God had forgotten them while they were slaves in Egypt?
Exodus 3:9 And now the cry of the Israelites has reached me, and I have seen the way the Egyptians are oppressing them.
Deuteronomy 26:7 Then we cried out to the LORD, the God of our ancestors, and the LORD heard our voice and saw our misery, toil and oppression.
If no one ever searched for God, then why do the scriptures say people did in the New Testament, when Jesus arrived, people shouted: “Son of David, have mercy on me”. They were looking for the Messiah. In the New Testament a blind man was searching for the Son of God, the prophet’s had said would come. I just showed you that people do search for God.
Again, Paul is speaking of a specific time when the Jews did really bad, and, it is to explain to the Gentiles that the Jews could be sinners just like them.
Paul goes on to explain how through Jesus now, no one is going to be justified by the purification works of the law anymore.
Now all have to go through Jesus to be purified.
We have to confess and repent of our sins to Jesus and believe his shed blood on the cross purifies us now and not the blood of lambs and the special dietary laws and the various external washings...
Jesus is the sacrificed Lamb once and for all.
Jesus does the purifying works for us now.
There were Jews who did not want to believe Jesus' blood is as the lamb's blood, and that they don't have to do that anymore.
Boasting is excluded means the Jews cannot boast anymore that they made people children of God's and brag about how many foreskins they got.
Galatians 6:13 Not even those who are circumcised keep the law, yet
they want you to be circumcised that they may boast about your circumcision in the flesh.