Question: I have recently become an evangelical and am now seriously studying the Bible for the first time in my life (though I was in the Roman Catholic Church for years). Can you interpret Colossians:1:24 for me, please? This passage appears to deny the efficacy of Christ's sufferings as the sole means of salvation.
Response: Colossians:1:24 in the KJV reads, "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."
This isn't speaking of working for Paul's own salvation. His "sufferings" were for "his [Christ's] body's sake." The very next verse begins, "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" (Col1:25).
Christ finished the work the Father had given Him to do (Jn:19:30). Consequently, everything that can be done for our salvation has been done. Nevertheless, according to Luke:4:18, Jesus said, "he hath anointed me to preach the gospel to the poor; he hath sent me to heal the brokenhearted, to preach deliverance to the captives, and recovering of sight to the blind, to set at liberty them that are bruised." Jesus did not personally do this for every person who was living then or who would be born. He is now seated at the right hand "of the Majesty on high" (Hebrews:1:3).
We have been commissioned (Mt 28:19, Mk 16:15, Lk 24:47, Jn 20; 21, Acts:1:8) to be His ambassadors; we will now "preach the gospel to the poor," etc. That is what's "lacking" in Christ's afflictions and why Paul was "made a minister."