What “is lacking in Christ's afflictions” according to Paul?

   Col 1:24 Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body’s sake, which is the church:

The KJ is more comfortable: what lacks are the afflictions of Christ in Paul's flesh. He has not yet had what he is going to get. He is taking the brunt of it so that others in the church don't have to.

v25 implies that his own sacrifice of his flesh is one of the marks of a minister of the gospel.

   Col 1:25 Whereof I am made a minister, according to the dispensation of God which is given to me for you, to fulfil the word of God;