Which concept is known as the "birthday" of the church?

The concept referred to as the "birthday" of the church is Pentecost. This event occurs fifty days after Easter and is significant in the Christian tradition because it marks the descent of the Holy Spirit upon the apostles and other followers of Jesus. This momentous occasion is described in the Book of Acts, where the apostles, filled with the Holy Spirit, began to speak in various tongues and courageously proclaim the message of Jesus Christ.

Pentecost is often considered the birth of the Christian church because it represents the moment when the followers of Christ transitioned from a small group of disciples into a community of believers empowered to spread the teachings of Jesus to the wider world. The transformative experience of receiving the Holy Spirit enabled them to carry out their mission, which led to the rapid growth of the early church.

In contrast, Easter commemorates the resurrection of Jesus, Christmas celebrates His birth, and Lent is a period of preparation leading up to Easter. While each of these reflects important aspects of the Christian faith, it is Pentecost that specifically signifies the establishment and empowerment of the church.