That translation make it look like the suffering was lacking in Christ's suffering and Paul had to finish it doesn't it? BAD TRANSLATION! When you read it in the Greek it's Paul who lacks the suffering that Christ endured and he rejoices in having to suffer some of those tribulations but certainly lacks the extent that Christ went through.
Colossians 1:24 I now rejoice in my sufferings for you, and do fill up the things lacking of the tribulations of the Christ in my flesh for his body, which is the assembly,
Colossians 1:24 I now rejoice in my sufferings for you, and do fill up the things lacking of the tribulations of the Christ in my flesh for his body, which is the assembly,
Thanks for the answer bear... Thought I would add the literal translation
Young's Literal Translation
I now rejoice in my sufferings for you, and do fill up the things lacking of the tribulations of the Christ in my flesh for his body, which is the assembly,