The Cause of Bahá'u'lláh is the same as the Cause of Christ

The Cause of Bahá'u'lláh is the same as the Cause of Christ. It is the same Temple and the same Foundation. Both of these are Spiritual Springs, the seasons of the soul-refreshing awakening and the cause of the renovation of the life of mankind. The spring of this year is the same as the spring of last year. The origin and end are the same. The sun of today is the sun of yesterday. In the coming of Christ, the divine teachings were given in accordance with the infancy of the human race. The teachings of Bahá'u'lláh have the same basic principles, but are according to the stage of the maturity of the world and the requirements of this illumined age 
- ‘Abdu'l-Baha  (Star of the West, vol. 19, no. 1, April 1928)

The ramification of two statements of Christ in the Bible: "Let the dead bury their dead”, and "That which is born of the flesh is flesh; and that which is born of the Spirit is spirit."

The world of humanity cannot advance through mere physical powers and intellectual attainments; nay, rather, the Holy Spirit is essential. The divine Father must assist the human world to attain maturity. The body of man is in need of physical and mental energy, but his spirit requires the life and fortification of the Holy Spirit. Without its protection and quickening the human world would be extinguished. Jesus Christ declared, "Let the dead bury their dead." He also said, "That which is born of the flesh is flesh; and that which is born of the Spirit is spirit." It is evident, therefore, according to Christ that the human spirit which is not fortified by the presence of the Holy Spirit is dead and in need of resurrection by that divine power; otherwise, though materially advanced to high degrees, man cannot attain full and complete progress. 
- ‘Abdu'l-Baha  (‘The Promulgation of Universal Peace)