  Before we ask what the quantum gravity theory is, it is a legitimate quest to
formulate a robust quantum field theory in curved spacetime (QFTCS). Several
conceptual problems, especially unitarity loss (pure states evolving into mixed
states), have raised concerns over several decades. Unitarity in QFTCS is
essential for formulating S-matrix and computing scattering amplitudes in
curved spacetime. In this paper, acknowledging the fact that {time} is a
parameter in quantum theory, which is different from its status in the context
of General Relativity (GR), we start with a "quantum first approach" and
propose a new formulation for QFTCS based on the discrete spacetime
transformations ($\mathcal{C}\mathcal{P}\mathcal{T}$) which offer a way to
achieve unitarity. Our framework for a QFTCS is built on a refined
understanding of standard QFT S-matrix that explicitly reflects the \cpt\,
invariance of scattering amplitudes. We rewrite the standard QFT equipped with
a direct-sum Fock space structure that is explicitly
$\mathcal{C}\mathcal{P}\mathcal{T}$, invariant. Applying this framework to
QFTCS, in the context of de Sitter (dS) spacetime, we elucidate how this
approach to quantization complies with unitarity and the observer
complementarity principle. We then discuss defining an S-matrix in de Sitter
spacetime that is analogous to Minkowski. Furthermore, we comment briefly the
implications of our QFTCS approach for quantum gravity.
