Thou therefore, my son, be strong in the grace that is in Christ Jesus.
Thou therefore - following my example (2 Timothy 1:8; 2 Timothy 1:12), and that of Onesiphorus (2 Timothy 1:16-18), make up by thy stedfastness for the faithlessness of those who forsook me (2 Timothy 1:15).
My son. Children ought to imitate their father.
Be strong, х endunamou (G1743)] - 'be inwardly strengthened,' 'be invested with power:' an abiding state of power, sure to manifest itself outwardly.
In the grace - the element IN which spiritual strength has place (cf. 2 Timothy 1:7).
That is in Christ Jesus - as its center and source to those in union with Him.