A first meaning of Jesus' instruction seems obvious: take nothing for the journey because you are to move freely and quickly. There is an urgency in the work you must do. You need to be unhampered, uncluttered, in order to do it effectively.
Another meaning sees this simplicity, even poverty, as essential for the credibility of a teacher or preacher. The Cynics, philosophers of the ancient world, supported their words with a lifestyle of dramatic simplicity and austerity. In the time of Saint Dominic, the Cathars of southern France lived lives of similar poverty and asceticism. Dominic realised that if Catholic preachers were to have any hope, they would have to embrace similarly poor and penitential ways. The credibility of their message depended on it. And was it not, in any case, simply returning to the simplicity of the first apostolic missions?
There is a danger in this of course: 'my guru is more ascetic than your guru'. Monastic communities were not always free of this kind of rivalry, as if asceticism were some kind of end in itself. What's the poverty for? what's the simplicity about?
The only acceptable answer in a Christian context is that it is in order to come closer to Christ, to imitate him more completely in how we live and work. The apostles are being formed for their mission. Christ instructs them not just through his words but through his lifestyle. Like him, they are to preach repentance, cast out demons, and anoint people with oil in order to heal them.
We may think that these things can still be done from a position of wealth and power. But the teaching and practice of Jesus, and the experience of the Church, says the opposite. A moment's reflection confirms that our own most convincing teachers of the gospel impressed us by their simplicity, their sincerity, their non-attachment to wealth or power. Their freedom in the service of truth enabled them to teach, and reconcile, and heal, as they did.
Christian simplicity needs to be physical and material, not just an idea or a notion. On the other hand it is never an end in itself. It serves the preaching of the Word, prepares the way for the power of the Word to show itself, and follows inevitably on our repentance and our acceptance of the Word.
We know from the other gospels that this experience of simplicity and its effectiveness filled the apostles with joy.