"Not that I have already obtained this or am already perfect, but I press on to make it my own, because Christ Jesus has made me his own." Philippians 3:12
That very statement is key to endurance and joy "Christ Jesus has made me his own". All my reaching and yearning and striving is not to belong to Christ (which has already happened), but to complete what is lacking in my likeness to him.
"For by a single offering [namely himself] he has perfected for all time those who are being sanctified" Hebrews 10:14
Passage taken from 'The Passion of Jesus Christ' (pages 48-49) by John Piper