I, humbly and honestly, don't think that this is what this text is saying.
Paul is talking, in context, about how HE has suffered for the cause of Christ, ... and how HIS suffering is added to Christ's suffering, to which will be added our own suffering.
How would our acceptance of the gift of salvation ... be filling up what is missing in the AFFLICTIONS of Christ ?
Note that Paul starts the passage saying, "Now I rejoice in MY SUFFERINGS for your sake, and fill up on my part that which is lacking of the afflictions of Christ IN MY FLESH for his body's sake ...
This is about PAUL'S SUFFERING, in HIS FLESH ... to fill that part which is lacking in the AFFLICTIONS of CHRIST ...
let's look at the context since you still are trying to turn it into a discussion about suffering not what is lacking.
24 Now I rejoice in my sufferings for you, and
I am completing in my flesh what is lacking in Christ’s afflictions for His body, that is, the church.
The colored part is the part in question...what is lacking if there is nothing lacking? Please keep the question in mind. Now look at the bold part...the church is what is lacking...but if we are right that the church is what is lacking we should be able to verify that understanding later in the passage...so let's look.
25
I have become its servant, according to God’s administration that was given to me for you, to make God’s message fully known,
a servant of what? God's message, right? This goes back to the accompanying passages I posted but we can circle back to them in a bit.
26 the mystery hidden for ages and generations but now revealed to His saints. 27 God wanted to make known among the Gentiles the glorious wealth of this mystery,
which is Christ in you, the hope of glory.
Now notice that it does not say that the mystery is Christ in you the assurance of glory but rather it says Christ in you the hope of glory...what does scripture tell us about hope? Romans 8:25 (HCSB) But if we hope for what we do not see, we eagerly wait for it with patience. Which would mean that our home in glory is not our salvation for we are still hoping for that...what then is our salvation? We are also still looking for confirmation as to what is lacking....
28
We proclaim Him, warning and teaching everyone with all wisdom, so that we may present everyone mature in Christ. 29 I labor for this,
striving with His strength that works powerfully in me.
See, Christ needed the church for two reasons, one to receive His gift of coming glory and secondly to share the gospel to the world...but notice what it says...we are striving with His strength...together, partnership, companions...we are working together in a symbiotic relationship with Christ. Which takes us back to the other passages I presented about working together with. Now...back to the topic as it applies to this passage. If there is nothing lacking in Christ's afflictions, why does Colossians say that the church was lacking and that we strive together with Him?
Please feel free to show in context as I have since context is important to rightly dividing the word as is the totality of scripture which I also provided for you.