Today's Gospel reading comes from what is called the longer ending of St. Mark's Gospel. The reason it is called this is because verses 9-20 of chapter 16 are not found in the earliest extant manuscripts. There is good evidence these verses were considered canonical in the second century, as writings from Justin Martyr and Tatian indicate. The church has included them as canonical, and we find strong consistencies with the other post-resurrection accounts and sources. One of the most obvious similarities is the call to proclaim the Gospel to the whole of creation.
What is entailed in proclaiming the Gospel? Ask 10 different Christians this question and you might get 10 different answers. Some might say preaching about sin and repentance. Some might say catechesis. Some might say charitable works. There are perhaps other options that might emerge. Yet, what is at the core?
To get at a good answer we need to ask ourselves what the Gospel really means. Gospel comes from the Greek word εὐαγγέλιον (transliteration: euaggelion, phonetic: yoo-ang-ghel'-ee-on). We tend to translate it into our modern English as "good news," but it more specifically means "glad tidings." This word was used in the ancient world to announce a military victory or successful conquest. Thus, I think in the New Testament context the Gospel of Jesus Christ is the glad tidings of the victory that He has won. It is the proclamation that Christ won over the powers of evil, darkness, and even death itself. The second Adam has come from heaven so that we shall also bear the image of the man of heaven instead of the image of the man of dust (1 Corinthians 15:48-49).
No longer would humanity be in perpetual bondage to sin and decay. No longer would malevolent spiritual powers hold sway over people and nations. In the fullness of time, the King set out and decisively defeated all those who oppose Him. By the power of Jesus' death, our sins are forgiven. By the force of His resurrection, we know our own bodily death is not the end.
To go into the whole of creation and proclaim the Gospel is to announce what Christ has done and to teach others about Him. It is about God keeping His promise and the upward thrust of human history toward union with God. It is the very best news imaginable.