The starting point for an existential response to the human condition is the here and now. Heidegger's term
Befindlichkeit covers it. We orient ourselves to this place and time.
This present circumstance. which is the point of departure. If we genuinely wish to hear the Truth, then we know we are being called to respond to this world of "negation and falsehood" and the challenge of transcending our own nature, which is subject to the "defilements of earthly vanities."
Arguably, the act of turning toward G-d and the decision to obey is the most fundamental existential choice. The choice is made in passionate commitment, which is substantially different from intellectual assent to ideology. In fact, ideology is thought to comprise "grievous veils" that must be eliminated in order to gain true knowledge:
We have driven from the human heart all else but Him Who is the Desire of the world, and glory therein. We cleave to no knowledge but His Knowledge, and set our hearts on naught save the effulgent glories of His light.
~Baha'u'llah, Kitab-i-Iqan
Kitab-i-Iqan (Book of Certitude)