The word ‘psychology’ comes from two Greek roots, ‘psyche’ meaning soul or spirit and ‘logia’ meaning the study of. Christianity can be thought to come from an historical, personal study of the soul, starting with Jesus of Nazareth and then continuing over the last 2,000 years.
If the description of Jesus in the Gospel accounts is somewhere near accurate, he spoke with ‘authority, unlike the scribes’ – which is to say, he had a firsthand understanding of various aspects and practices related to the soul or spirit. His study was not academic but practical and personal. Nowadays, the equivalent of ‘scribes’ could perhaps best be academics, such as philosophers and theologians, who engage in logic chopping with little personal experience of the issues at hand. Since then, various practices have developed within Christianity which have also been based on firsthand experience related to the spirit.
If Christianity is essentially psychology derived from firsthand experiences and then applied to various practices, then you can see why defining Christianity doctrinally or theologically is misleading, and why arguing about whether Christianity is true or not based on abstract, derivative theological arguments is to a significant degree beside the point of what Christianity is.