Ritual Knocking

The ceremonial entry into the church and to the altar in the sanctuary is also acted out in the ritual of the opening of the Holy Door, the Porta Santa at St. Peter's Basilica in Rome and other carefully selected Churches. This rite is executed only every twenty-five years and represents the entry of the children of God into the presence of the Lord. Medieval medals struck for the occasion often show Christ on one side of the portal and the pope or the people on the other. The pope knocks three times with a golden hammer upon which the door is opened by the masons and he may enter through it and proceed to the sanctuary. The remainder of the clergy and the people then follow after him. A prayer said by Pope Clement VIII during the rite in 1600 demonstrates clearly that the ceremony does indeed portray entry into the temple of God. "Open unto me the gates of justice. When I am entered. I will praise my Lord. I will enter, O Lord into Thy house. I will adore thee in Thy fear in Thy temple." (The Catholic Liturgy 

and the Mormon Temple by Marcus von Wellnitz, pg. 32)

From the portico, you enter the Basilica through five heavy bronze doors: the one on the far right is called the "Porta Santa," the Holy Door, which is only opened during the jubilee year. It is normally walled up from the inside, and is only open during Holy Years - last during the Jubilee Year 2000. The Holy Doors found in some cathedrals symbolize the threshold that has to be crossed in order to achieve salvation.

"I am the door: by me if any man enter in, he shall be saved..."(John 10:9). It is opened by the Holy Father during a solemn ceremony, and closed at the end of the Holy Years. During that time, thousands of pilgrims enter the basilica through that door to obtain indulgences. The two stone plaques above the door are in memory of the Jubilee of 1975 and the extraordinary Jubilee of Redemption in 1983. The Holy Door was decorated for the 1950 Jubilee by Vico Consorti with 16 bronze panels.
Holy Door (Porta Santa in Italian and Porta Sancta in Latin)

Hugh Nibley notes: "how the bishop leading the prayer circle in the Syriac Testament of Our Lord "stands with upraised hands and offers a prayer at the veil," after which he proceeds "to make the sacrifice, the veil of the gate being drawn aside." St. Augustine's version of the Priscillian prayer circle ends with the apparently incongruous statement, "I am the Gate for whoever knocks on me," which Augustine explains in terms of Psalms 24:7, referring to the veil of the temple." [Letters 237]. (John A. Tvedtnes; Temple Prayer in Ancient Times)

See Also:
Knocking 3 times on the Holy Door
The Holy Doors
Panels of the Holy Door