St. Mark, whose feast we celebrate today, was not one of the apostles.
Nonetheless, he was one of the disciples of St. Peter, and he was mentioned in the 1st reading as a "spiritual son" of St. Peter.
He could also possibly be one of the followers of Jesus and later on became a disciple of St. Peter and he eventually wrote the account of the life and ministry of Jesus, which became one of the gospels of the Church.
In his gospel, St. Mark captured the core and essence of the ministry and message of Jesus.
As we heard it in the gospel, and what is often called the "Great Commissioning", the way St. Mark puts it across straight and sharp in just a few words.
"Go out, proclaim the Good News, believe or be condemned, cast out devils, work signs and miracles, heal the sick".
To understand and accept the Good News and the commissioning of Jesus, we need to "Go out" like St. Mark did.
Because when we are out there, and there is no dependency and certainty but Jesus Himself, then we will truly experience the power of the Good News and become true disciples of Jesus Christ.