Mainstream scholars acknowledge that the Trinity is not explicitly defined in the New Testament but was formulated later by combining various verses and theological concepts.
Scholars freely admit that the Trinity is never presented as a complete doctrine in the New Testament, but rather is built from scattered Bible verses.
In the New Testament there is no explicit statement of the doctrine
Evangelical Dictionary of Theology (p. 1112)
It is not a Biblical doctrine in the sense that any formulation of it can be found in the Bible
New Bible Dictionary (p.1221)
The New Testament does not present a systematic presentation of the Trinity. The scattered segments from various writers that appear throughout the New Testament reflect a seemingly accepted understanding that exists without a full-length discussion.
Holman Bible Dictionary (p.1372)
Theologians admit the New Testament does not define the Trinity. Furthermore, there is no clear teaching that salvation depends on the belief in the Trinity.