But God forbid that I should boast, except in the cross of our Lord Jesus Christ
Paul was glad that Jesus of Nazareth died! Not because he hated Him, that had been the case when he was the fanatical, persecuting Pharisee - he was glad that Jesus had been crucified - he thought that was the end of Him. But then he met Him! On the Damascus Road he encountered the risen Jesus, and owned Him as Lord and trusted Him as Saviour.
You might have thought it would have made him sorry that Jesus died, but not at all! In fact he boasted and gloried in the cross of Christ! Why? Because it was the means of salvation for guilty sinners.
It is so vital that we all learn that the Lord Jesus was not just treated by the Romans as if He was guilty, but when He was on the cross He was treated by God as if He was guilty, when God laid on Him the punishment due to sin (see Isaiah 53 v 5-6).
Prior to Paul's conversion his boast would have been in the life he lived, but he came to realise that the life he lived fell short of God's standard and could never gain him acceptance with God. From his experience of conversion Paul's boast was no longer in the life he lived, but in the death Christ died, because that pays for all the sins of the life he lived, and it can do the same for you.
No one in heaven will be able to boast about what they did to get there. Every one of them will be boasting in what Christ did on the cross. Make Christ your trust, and His cross your boast today.