"Transmit" means literally to "send-across"—to send across from person to person, yes, but also and even more so to send across from generation to generation. In other words, if the Great Commission is the Lord's mission extended through space, the Great Transmission is the mission extended through time. It's about handing the faith down to our descendants, and its focus is therefore on the "teaching to keep all that I have commanded you" portion of Jesus' mandate.
The book of Psalms picks up on the Great Transmission:
"[God] established a testimony in Jacob
and appointed a law in Israel,
which he commanded our fathers
to teach to their children,
that the next generation might know them,
the children yet unborn,
and arise and tell them to their children,
so that they should set their hope in God
and not forget the works of God,
but keep his commandments" (Psalm 78:5–7),
The Great Commission strives to keep faith in this generation; the Great Transmission strives to keep faith for the next.
To be clear, God's mission isn't a matter of either Commission (reaching the lost) or Transmission (teaching the found); it's both/and. The overarching mission to "make disciples" comprises both these components, and we neglect one or the other at our peril.
In my experience, though, transmitting the faith tends to be regarded as second-rate ministry, whereas propagating it is seen as more urgent—if not more faithful. Evangelism is the intrepid Jeep of Christianity, traversing across terrain to bring the good news; education is the ho-hum minivan of the Church, merely ferrying the faith across generations. (Incidentally, my family has both a Jeep and a minivan. Like I say, it's not either/or people.)
Furthermore, in the last few decades within American Christendom there has been so much emphasis on outreach that catechesis and Christian formation has suffered. As a result, as research from social scientists has borne out, we have a generation of Christians who largely don't know what they believe or why. We need to check that in our zeal to invite more people to the campfire, so to speak, we don't neglect to fan the flame and so let it burn out.
In Sunday's sermon I referenced Cormac McCarthy's novel The Road, and its idea of "carrying the fire" (you can see a related scene from the film adaptation here). Carrying the fire, in the book, is a metaphor for passing on the deep-seated hope of humanity—what one of the characters calls "the breath of God." Without that fire, hope is smothered.
We live in dark and anxious times. It's not quite so bleak as depicted in the post-apocalyptic setting of The Road, thankfully, but there's no question that amid the wind and wave of the world the flame is prone to flicker. The Great Transmission is about carrying the fire of faith for the sake of the next generation, and ensuring that in every age the world might see and be warmed by its true and lasting Light.