That is only what agnostics believe, but that does not mean that the existence of God is unknowable, or that the the claims of any "messenger" or scripture about God are necessarily all baseless crap.
God is not entirely unknowable.
"God communicates his will and purpose to humanity through intermediaries, known as
Manifestations of God, who are the prophets and messengers that have founded religions from prehistoric times up to the present day.
[5]
While God's essence is inaccessible, a subordinate form of knowledge is available by way of mediation by divine messengers, known as
Manifestations of God.
The Baháʼí teachings state that God is too great for humans to create an accurate conception of. In the Baháʼí understanding, the attributes attributed to God, such as
All-Powerful and
All-Loving are derived from limited human experiences of power and love. Baháʼu'lláh taught that the knowledge of God is limited to those attributes and qualities which are perceptible to us, and thus direct knowledge of God is not possible. Furthermore, Baháʼu'lláh states that knowledge of the attributes of God is revealed to humanity through his
messengers.
[12] "
From:
God in the Baháʼí Faith