A deacon is an ordained member of the clergy in the Catholic Church, although he is not a priest. The word deacon comes from the Greek word for messenger or servant. He can impart blessings, administer Baptism, and preside at weddings. He also takes on a special role in serving the priest at Mass and can even give the homily; however, his most important duty at Mass is to proclaim the Gospel. By tradition, the ministry of preaching the Gospel belongs to the Deacons. He cannot consecrate the bread or wine into the Body and Blood of Christ and he cannot hear confession or absolve sins.
A longstanding sacred tradition
In the Acts of the Apostles, St Luke writes about the ordination of the first seven deacons,
“In those days, as the number of disciples was increasing, there occurred a murmuring of the Greeks against the Hebrews, because their widows were treated with disdain in the daily ministration. And so the twelve, calling together the multitude of the disciples, said: “It is not fair for us to leave behind the Word of God to serve at tables also. Yet truly, we will be continually in prayer and in the ministry of the Word.” Therefore, brothers, search among yourselves for seven men of good testimony, filled with the Holy Spirit and with wisdom, whom we may appoint over this work. And the plan pleased the entire multitude. And they chose Stephen, a man filled with faith and with the Holy Spirit, and Philip and Prochorus and Nicanor and Timon and Parmenas and Nicolas, a new arrival from Antioch. These they set before the sight of the Apostles, and while praying, they imposed hands on them.” -Acts 6:1-6
Proclaiming the Word
When the pope is celebrating the Mass and a deacon is with him, the deacon proclaims the Gospel. That is how important this tradition is! It is spelled out for us in the General Instruction of the Roman Missal:
By tradition, the function of proclaiming the readings is ministerial, not presidential. The readings, therefore, should be proclaimed by a lector, and the Gospel by a deacon or, in his absence, a priest other than the celebrant. –USCCB G.I.R.M. paragraph 59
As Clergy
Every pope, bishop, and priest was first a deacon through Holy Orders. In fact, all priests serve one year(sometimes more, sometimes slightly less in grave circumstances at the discretion of the local bishop) as an ordained transitional deacon. Others are permanent deacons who are ordained to the diaconate but are not going to be ordained to the priesthood.
As pope, bishop, or priest, each of those men hold the fullness of the diaconate already. So then, it raises the question, why does the deacon have to proclaim the Gospel if a priest is celebrating the Mass?” There are two points to that. The priest does proclaim the Gospel in the absence of a deacon. However, the priest’s main duty is to preside at Mass and the function of proclaiming the Gospel is ministerial in nature(Linked is an example of the Diocese of Springfield in Illinois reaffirming the Deacon’s role).
You can find more information on the Diaconate and requirements to be considered as a candidate here. You should contact your local Diocese for inquiries about becoming a deacon in your own diocese. Most diocese have a page on their vocation’s website about the diaconate(Diocese of Arlington, Archdiocese of Washington, Archdiocese of New York, etc)
Below is a video of three deacons proclaiming the Gospel at St. Peter’s Basilica on Good Friday.
A.M.D.G.
Sed Sensus Profundior 