Paul knew because Paul experienced it. He mainly knew that he didn’t need to be God.
Imagine that you’re in prison and there’s a group of people out there that you are concerned about even from a distance. It appears that Paul had never been to this church in Colossae but rather sent Epaphras to preach the gospel and perhaps found the church. Epaphras continued to give Paul reports as well as be an intercessor for the people in his hometown. Paul was concerned about their maturity in Christ and susceptibility to false doctrine but he couldn’t get to them as he was “in chains.”
No problem. Paul knew his God. And so he prayed. He asked God to teach them himself. He reports, “we have not stopped praying for you and asking God to fill you with the knowledge of his will through all spiritual wisdom and understand” (Col 1:9).
If Paul couldn’t be there, he would trust the Lord to do it. And he knew it was possible. Being divinely instructed was something we read about in Galatians that Paul himself experienced. For 14 years after his conversion Paul had pulled himself away and was taught by the Lord (Gal 2). I love this. I love it because it means we can pray that the Lord himself would teach people even when we’re not in the mix. We can pray for closed countries that the Lord himself would evangelize even when we have trouble getting past borders (and he does through dreams and visions). And we can do our part in prayer even when we can’t be there in person. This, my friends, is encouraging. Now who can I go pray for?