There are actually two parting mandates from Jesus to HIS first generation disciples to us today. There is the “preaching” mandate found in Mark 16:15 with helpful conditions following, as in verses 16-18,& 19.
There is the “teaching” mandate found in Matthew 28:20. Both mandates are equally valid and helpful to the other. To preach (kerysso) is to strongly urge acceptance of the message with clearly defined consequences for not doing so. It indicates a call for an immediate response.
To teach is to provide all the information necessary to bring the listening followers into their place of proper leadership so that they can help bring their own people into the divinely assigned parameters of discipleship. Of course, the latter is years in the making with many possible setbacks. The latter (discipleship teaching) is focused on developing national leaders, who have much spiritual substance to pass onto their own disciples.
This follows along the lines of 2nd Timothy 2:2. If all goes well with the latter, there will be many ripple effects of continual discipleship teaching cycles. The latter can involve Bible Colleges and ministerial leadership conferences. There is and will continue to be an increase in efficiency in the latter using today’s technology of Skype and other type of receiving sets.