In today's Gospel, we read St. Mark's account of Jesus giving the Great Commission to His disciples. What I take from these passages is an unqualified boldness; the Gospel will go forth into all regions of the earth despite the inherent dangers in that exercise. This reminds us of the confession of St. Peter after which Jesus tells him that the "gates of hell shall not prevail against [Jesus' church]."
We can be tempted to take this in a defensive way, wherein the besieged Holy City of God will not be breached. But I think the Lord Jesus means to tell St. Peter, and us, that we are to be on the offensive. We will smash through the gates of hell, obliterating the powers of evil and death by the power of Christ. This is what I find in today's Gospel reading. Despite great effort, the enemy will not prevail. No demonic power, no physical danger, will thwart the Good News and the consummation of Christ's Kingdom. The onward march upward and onward continues unabated by the opposition.