He’s drawing a distinction between the ought of v3 where the Colossians are asked to pray for the apostle that he might make the gospel known as he ought to speak and how Paul instructs the Colossian believers in v6 how they ought to answer each person.
“We may describe this difference by saying that while the apostle looks for many opportunities for direct evangelism and teaching, the typical Christian in Colossae is to look for many opportunities for responsive evangelism.
If this distinction is a correct one, it immediately commends itself by its sanity and realism. Harm can be done by sincere believing people who feel compelled to preach and testify to those with whom they mix in shop or office. Rightly aware of the importance of their message, the sad ignorance of many of their neighbours, and the urgency of the times, they plunge in bravely (whatever the temperature). But direct assault on entrenched apathy (to change the metaphor) is seldom successful and can never be carried out by normally sensitive people without great cost to nerve and confidence. Alas, one consequence of failure in such verbal witnessing is a discouragement sufficiently severe at times to lead to disengagement from this part of the battle altogether.
Now Paul’s advice to the Christians is not along the lines of possessing oneself of better techniques with which to approach people. Rather he turns the problem right around so that the Christians can see their responsibilities in a much more promising light. Their privilege, simply put, is to answer everyone. That is to say they are to respond to the questions of others rather than initiate conversations on leading topics; they are to accept openings rather than make them.
This is, emphatically, not to sound the retreat. Paul evidently believes that opportunities for response and explanation are to be found everywhere, for everyone is looking to discover answers about life and its meaning. And Paul evidently thinks that believing Christians should be found everywhere too, ready to take up these frequent opportunities.
It is obvious what strain this removes from conscientious Christians. The pressure to raise certain topics and reach certain people can make it difficult to live or talk normally. In any case, we go to the office to work, not evangelize. But by being ready and willing to respond the way is opened to a more serene, and successful, approach to each day’s opportunities. It opens the way too, for a greater dependence on God’s leading as well as for a more relevant and sensitive witness, suited to each individual. And remember, when the outsider has chosen the time and the place and the subject, how wonderfully free is the Christian to ‘open his mouth’ and tell ‘the good news of Jesus’.”