The principle of diplomatic immunity dates from Antiquity, when the Greek government extended special status to foreign envoys. Basically, it allows representatives of foreign governments to work and operate under the laws of their home country while abroad.
In a sense we as Christians have diplomatic immunity on this earth.
1 Therefore, there is now no condemnation for those who are in Christ Jesus…
No condemnation means a not guilty verdict. So does this give us a license to sin if we are in Christ Jesus?
It is always necessary to read verses in context:
2 because through Christ Jesus the law of the Spirit who gives life has set you free from the law of sin and death.
We are, through the blood of Jesus Christ set free the law of sin and death. This means that we have diplomatic immunity from the penalty from sin. We are set free from eternal death!
But this definitely does not give us a license to sin! God is quite clear on what will happen with us when we still choose to sin after we received diplomatic immunity through Jesus Christ:
19 Those whom I love I rebuke and discipline. So be earnest and repent.
A good father will discipline his children. In the same way our heavenly Father will discipline us when we sin.
So our sin will not condemn us to eternal death, but if you keep on sinning you will be disciplined by a loving Father.