"Paul’s logic runs like this: you have been spiritually “circumcised.” This circumcision took place when you were buried with Christ and raised with him. And this burial and resurrection with Christ happened when you were baptized. As this paraphrase of Paul’s argument also reveals, the popular explanation that Paul uses baptism as a symbol of our death to the old life (when we are plunged beneath the water) and resurrection to the new life (when we arise out of the water) is also wide of the mark. Baptism does not symbolize what happened when we were converted; it somehow is integrally involved in that conversion itself. The best way to account for this and at the same time to do justice to Paul’s constant emphasis on our faith as the key to our coming to Christ (as he does at the end of this very verse, as if to guard against a possible misunderstanding) is again to recognize a broadly attested New Testament theological concept dubbed by James Dunn “conversion-initiation.” The New Testament connects our coming to Christ (being converted and initiated in the new covenant community) to faith, to repentance, to the gift of the Spirit, and to water baptism, in various combinations. Any of these, in a kind of metonymy, could be used to connote the whole experience–implying, of course, in each instance, the presence of all the others. Water baptism, then, as a critical New Testament rite intimately connected to our conversion experience, could be used as shorthand for the whole experience."
Douglas Moo, The Letters to the Colossians and to Philemon, p. 202