Complex sentence: Later, in the Christian liturgical year, it became a feast commemorating the descent of the Holy Spirit upon the twelve Apostles and other followers of Jesus Christ, as described in the Acts of the Apostles 2:1--31.
Split into simpler sentences: Later, in the Christian liturgical year, it is also a feast commemorating the descent of the Holy Spirit upon the twelve Apostles of Christ. Among Christians, Pentecost commemorates the descent of the Holy Spirit upon the Twelve Apostles and other followers of Jesus as described in the Acts of the Apostles 2:1--31.