From the Baha’i Writings:
Giving to the Fund, therefore, is a spiritual privilege, not open to those who have not accepted Bahá’u’lláh, of which no believer should deny himself. It is both a responsibility and a source of bounty. This is an aspect of the Cause which, we feel, is an essential part of the basic teaching and deepening of new believers. The importance of contributing resides in the degree of sacrifice of the giver, the spirit of devotion with which the contribution is made and the unity of the friends in this service; these attract the confirmations of God and enhance the dignity and self-respect of the individuals and the community.
(From a letter of the Universal House of Justice) (Lights of Guidance, p. 251)