The apostle Paul could have used many prepositions, but he used σὺν, and in doing so, it is hard to fathom he meant anything but a literal presence with those in Colosse. Of course, when I say “literal,” I do mean literal, but not literally present in the body. Rather, he was literally present in the spirit.
The means by which this presence is achieved, though he be not with them in his body, is by means of the spiritual unity of the body of Christ. By being in Christ, Christians are united spiritually to one another by means of the Holy Spirit.1
For this reason, on the road to Damascus, the Lord Jesus asked Paul, “Why did you persecute me?”2 Did the apostle Paul ever persecute the Lord Jesus himself? Certainly not. But, the apostle Paul did persecute Christians, those who are “in Christ,” and by doing so, he persecuted Jesus, too, for Christians and Jesus Christ himself are united spiritually. For this reason, the Lord Jesus said, “...inasmuch as you did it to one of the least of these my brethren, you did it to me.”3 Indeed, the apostle Paul said that Christians in the body of Christ are “body parts of one another.”4 Therefore, “if one body part suffer, all the body parts suffer with it...”5
Because of this organic unity, the apostle Paul was literally present in the spirit with those in Colosse, just as those in Colosse were present with Paul, and every Christian present with one another throughout the entire world, by means of the Holy Spirit who unites all Christians.
1 1 Cor. 6:17
2 Acts 9:5
3 Matt. 25:40
4 Rom. 12:5
5 1 Cor. 12:26