The solemn Gospel procession has rich symbolism rooted in a reverence for the Word of God.
In the Roman Rite of the Catholic Church, there is a procession of the Gospel that occurs on more solemn celebrations. If enough servers are present, the Gospel is accompanied by two lighted candles and a thurible holding incense.
This tradition has a long history, dating possibly to the 3rd or 4th centuries. Nikolaus Gihr, in his book The Holy Sacrifice of the Mass, explains the symbolism of the candles.
St. Jerome already defended the higher meaning of this very ancient custom of lighting candles at the Gospel, inasmuch as he insists that thereby we should give expression to the joy and jubilation of our hearts at the good tidings of salvation. — Above all, the light by its brightness and its glow symbolizes Jesus Christ, the Sun that knows no setting and the Light of the City of God as well on earth as in heaven. By means of the Gospel Christ is the light of the world; by the Gospel God has called us to the wonderful light of Christian truth and grace. In this dark vale of the earth “Thy word is a lamp to my feet, and a light to my paths” (Ps. 118, 105)…From the word of God beams a secure light to guide us amid the various directions and helps, as well as amid the various wants, obstacles and dangers we meet on this path so stern and so difficult to be determined.
The presence of candles at the Gospel also heightens our awareness at Mass, alerting us to something important that is happening in the sanctuary. The Gospel contains the words of Jesus himself and so we are exhorted to pay special attention to his message.
With regard to incense, the symbolism is also rich and points to a deeper spiritual reality.
In the first place, the incensing of the Book of the Gospels is to be regarded as an act of holy reverence, a religious mark of honor paid to “the words of eternal life,” which the Lord here speaks to us. — The fragrant clouds that envelop the book call to mind how by the announcement of the Gospel is spread abroad and around the good odor of the pre-eminent knowledge of Jesus Christ…The incense furthermore admonishes us, with what heavenly ardor of devotion the words of the Gospel should be announced by the deacon and also by the priest, and be listened to by the faithful and laid up in their hearts. — As the bright flame of the lighted taper is an image of a pure life, so the sweet fragrance of incense also symbolises, finally, a virtuous, God-fearing life…Virtue, indeed, exhales a sweet and a refreshing perfume; to prove this, the Lord has often wonderfully effected that the bodies of the saints in their life-time or after their death exhaled a sweet scent, altogether supernatural and heavenly.