What is the changeless commandment of God?

Question : What is the changeless commandment of God?

Indeed thou didst grasp the significance of rendering assistance unto God and didst arise to achieve this through the power of wisdom and utterance. Say: To assist Me is to teach My Cause. This is a theme with which whole Tablets are laden. This is the changeless commandment of God, eternal in the past, eternal in the future. Comprehend this, O ye men of insight. They that have passed beyond the bounds of wisdom fail to understand the meaning of assisting God as set forth in the Book. Say: Fear ye God and sow not the seeds of dissension amongst men. Observe ye that which hath been enjoined upon you by your Lord, the Almighty, the All-Knowing. He knoweth the reality of victory and hath taught it to you with an utterance that the vain imaginings of them that rove distraught in the wilderness of doubt can never corrupt.

                (Bahá'u'lláh, Tablets of Bahá'u'lláh, p. 196)

Question. Why is the changeless commandment of God to teach His Cause?

Say: If it be Our pleasure We shall render the Cause victorious through the power of a single word from Our presence. He is in truth the Omnipotent, the All-Compelling. Should it be God's intention, there would appear out of the forests of celestial might the lion of indomitable strength whose roaring is like unto the peals of thunder reverberating in the mountains. However, since Our loving providence surpasseth all things, We have ordained that complete victory should be achieved through speech and utterance, that Our servants throughout the earth may thereby become the recipients of divine good. This is but a token of God's bounty vouchsafed unto them. Verily thy Lord is the All-Sufficing, the Most Exalted.

    (Baha'u'llah, Tablets of Baha'u'llah, p. 197)