The Trinity is not explicitly and formally a Biblical Belief

"The trinity of God is defined by the Church as the belief that in God are three persons who subsist in one nature. That belief as so defined was reached only in the 4th and 5th centuries AD and hence is not explicitly and formally a biblical belief." --The Dictionary of the Bible, John L. McKenzie, S.J., p. 899

"It is difficult in the second half of the 20th century to offer a clear, objective and straightforward account of the revelation, doctrinal evolution, and theological elaboration of the Mystery of the trinity. Trinitarian discussion, Roman Catholic as well as other, present a somewhat unsteady silhouette. Two things have happened. There is the recognition on the part of exegetes and Biblical theologians, including a constantly growing number of Roman Catholics, that one should not speak of Trinitarianism in the New Testament without serious qualification. There is also the closely parallel recognition on the part of historians of dogma and systematic theologians that when one does speak of an unqualified Trinitarianism, one has moved from the period of Christian origins to, say, the last quadrant of the 4th century. It was only then that what might be called the definitive Trinitarian dogma 'One God in three Persons' became thoroughly assimilated into Christian life and thought. . .it was the product of 3 centuries of doctrinal development" --The New Catholic Encyclopedia, Volume XIV, p. 295

"Because the Trinity is such an important part of later Christian doctrine, it is striking that the term does not appear in the New Testament. Likewise, the developed concept of three coequal partners in the Godhead found in later creedal formulations cannot be clearly detected within the confines of the canon. . . . While the New Testament writers say a great deal about God, Jesus, and the Spirit of each, no New Testament writer expounds on the relationship among the three in the detail that later Christian writers do." --The Oxford Companion to the Bible, Bruce Metzger and Michael Coogan, p. 782

"In Christian doctrine, the unity of Father, Son, and Holy Spirit as three persons in one Godhead. Neither the word Trinity nor the explicit doctrine appears in the New Testament, . . . The Council of Nicaea in 325 stated the crucial formula for that doctrine in its confession that the Son is 'of the same substance [homoousios] as the Father,' even though it said very little about the Holy Spirit. Over the next half century, Athanasius defended and refined the Nicene formula, and, by the end of the 4th century, under the leadership of Basil of Caesarea, Gregory of Nyssa, and Gregory of Nazianzus (the Cappadocian Fathers), the doctrine of the Trinity took substantially the form it has maintained ever since." --The Encyclopaedia Britannica, on the heading "Trinity"