A person can experience a sense of “one-ness with the universe” as an altered state of consciousness through many ways via drugs, meditations, stress, mentally ill [schizo, epileptic, etc.] and even brain damage [note Jill Bolte - a neuroscientist].
What is critical to the above experience is how one can apply to optimize one’s well being. The 4NT-8FP life problem-solving technique provide one the path to such an optimization.
Buddhism has the principle of the Middle-Path to ensure one do not get stuck in one extreme, thus via the Two-Truths principles holds there is oneness with the universe [non-duality] in one perspective while in another perspective one is independent within the universe [duality].
There is this neo-advaita movement [of madness];
Neo-Advaita, also called the Satsang-movement[1] and Nondualism, is a New Religious Movement, emphasizing the direct recognition of the non-existence of the “I” or “ego,” without the need of preparatory practice. - Wiki
who keep preaching and telling the world there is no-you, no-me, no-self and all sort of one-sided extreme thus the call to abandon what we would term as the ordinary real world.
In normal everyday life, a Buddhist will acknowledge the independent external world. However where the impulses of the existential crisis is active, the Buddhist adopt the oneness with the universe, i.e. there is no independent God to offer salvation to the independent individual.
Since in this perspective of oneness, there is no independent soul to be lost and need to be saved, thus avoiding the sufferings from any “imagined” threat of loss upon mortality.
To be in state of holding to two extremes at the same time but not in the same perspective require one to cultivate the necessary skills supported by the effective algorithm within one’s brain/mind via proper meditations [a requisite of Buddhism] and reflections.
In general most of those who experienced an altered states of consciousness of one-ness via other means [drugs, etc.] do not convert their experiences to the practicals of life.
In the case of Buddhism, the ordinary believer do not start with an expectation of an experience of one-ness but rather such an experience of oneness spontaneously emerged after some time in the course of practicing Buddhism and reflections on its doctrine.
The problem with the above is at present there are so many schools and sects of Buddhism promoting a range of confusing and opposing interpretations of Buddhism-proper.
Thus there is a need to get to what is Buddhism-proper which can be extracted from the teachings of the Buddha in alignment with universal philosophy-proper.