What is the proper mode or way to show or acknowledge that one believes in God, the One Supreme Original Cause?
In short, one acts in such a way that they see things in connection to god
BG 4.35: Having obtained real knowledge from a self-realized soul, you will never fall again into such illusion, for by this knowledge you will see that all living beings are but part of the Supreme, or, in other words, that they are Mine.
since there are different grades of practitioners, this said action can be expected to be different
there are three general grades of practitioners (and also sub-grades within these grades .. but just to keep things simple)
1 - SB 11.2.47 "A person who is very faithfully engaged in the worship of the Deity in the temple, but who does not know how to behave toward devotees or people in general is called a kaniṣṭha-adhikāri."
2 - SB 11.2.46 "The madhyama-adhikārī is a devotee who worships the Supreme Personality of Godhead as the highest object of love, makes friends with the Lord's devotees, is merciful to the ignorant and avoids those who are envious by nature."
3 - The final type is a little bit more difficult to appreciate - basically they are on a level where there is no distinction between this temporary life and eternal life
SB 11.2.45: Śrī Havir said: The most advanced devotee sees within everything the soul of all souls, the Supreme Personality of Godhead, Śrī Kṛṣṇa. Consequently he sees everything in relation to the Supreme Lord and understands that everything that exists is eternally situated within the Lord.
Simply by
seeing a person of this callibre can be sufficient for one to become devoted to god - such an example would be someone like jesus.
negotiating all these differences can be difficult, but this is the general guideline
NoI 5: One should mentally honor the (Kannistha), one should offer humble obeisances to the (Madhyama), and one should associate with and faithfully serve that Pure devotee (Uttama) who is advanced in undeviated devotional service and whose heart is completely devoid of the propensity to criticize others.
It is strongly recommended that a practitioner comes at least comes to the platform of madhyama - as you can imagine, too many kannisthas causes problems