As a summary of the gospel Paul writes: ‘Jesus was delivered over to death for our sins, and raised to life for our justification’ (Romans 4.25).
Notice the connection between his resurrection and our justification.
To justify something is to declare it to be right. In an exam we might be challenged to ‘justify our answer’, in other words, to show it to be right. Paul is speaking of being justifed by God: as far as God is concerned, we are entirely in the right. There is nothing to be said against us.