After the ascension of Jesus Christ, the tenth day came: it was the fiftieth day after the Resurrection of Christ. The Jews had the great feast of Pentecost in commemoration of the Sinai legislation. All the apostles, together with the Mother of God and with other disciples of Christ and other believers, were unanimously in the same upper room in Jerusalem.

The Holy Spirit, according to the promise of the Savior, descended on the apostles in the form of tongues of fire, as a sign that He gave the apostles the ability and strength to preach Christ's teaching to all nations; He came down in the form of fire as a sign that it has the power to scorch sins and cleanse, sanctify and warm souls.

In Jerusalem at this time there were many Jews who came from different countries for the feast. The apostles went out to them and began to preach the risen Christ in their native languages. The sermon had such an effect on those who listened to it that many believed and began to ask: "What then should we do?" Peter answered them: "Repent and be baptized in the name of Jesus Christ for the forgiveness of sins, then you will also receive the gift of the Holy Spirit."

Those who believed in Christ willingly accepted baptism; there were about three thousand such people that day. Thus, the Kingdom of God began to be established on earth, that is, the Church of Christ.


Pentecost
Orthodox Christians on this day decorate houses and churches with green birch twigs and flowers.

This custom goes back to the Old Testament Church, when houses and synagogues were decorated with greenery at Pentecost in memory of how at Mount Sinai everything bloomed and greened on the day when Moses received the tablets of the law. The upper room of Zion, where the Holy Spirit descended on the apostles, at that time, according to the general custom, was also decorated with tree branches and flowers.
The flowering branches also remind us that under the influence of God's grace, human souls flourish with the fruits of virtue.