Never was there a more deceptive doctrine advanced than that of the Trinity. It could have originated only in one mind, and that the mind of Satan the Devil (Reconciliation [Watchtower Bible and Tract Society, 1928], p. 101).
Since the beginning of human history, the nature of God (i.e., how He revealed Himself) has been furiously attacked (esp. ontological monotheism). Though, one of the first heresies that emerged in first century church was that of the Judaizers. And the second heresy that the early church dealt with was that of the Gnostics. Both of which were thoroughly refuted by the apostles in there writings.
Jesus was clear on the subject: eternal life is to have “knowledge” of the true God (cf. John 17:3; 8:24). And Scripture presents that there is one true God who revealed Himself in three coequal, coeternal, and coexistent *distinct* persons—thus, God is Triune. The biblical data is undeniable. But many today (and historically) deny, in some way, shape, or form, the doctrine of the Trinity. We are not speaking of some peripheral, non-essential doctrine here: The belief in the doctrine of the Trinity is essential to ones salvation, for it is how God revealed Himself—the very nature or essence of His essential Being, the only true God.
If one removes the Son from the Trinity (in any way), the Son is reduced to either to a created being (as with, for example, Oneness believers and Jehovah’s Witnesses [JWs]) or the Son becomes a “separate” God (as in Mormonism). The Trinity is the biblical explanation of how there is one God and yet the Son is presented as both Creator and “God” (theos) distinct from the Father and Holy Spirit who are likewise presented as God.
Main Objections to the Trinity
Also, see JWs_Objections in which deals specifically with the Trinitarian objections made by the JWs. And see here Oneness_Objections in dealing with some of the specific Trinitarian objections made by the Oneness Pentecostals.
However, virtually all anti-Trinitarians utilize the same arguments as delineated below:
The Trinity is 3 separate Gods
This is a typical straw man argument that misrepresents the doctrine of the Trinity by assuming that “Trinity” means, three “separate” Gods. The very foundation, however, for the doctrine of the Trinity is *monotheism*: there exists only one true God (one Being, not one Person). As seen, the doctrine of the Trinity states that there are three *distinct* coequal, coeternal, and coexistent Persons who share the nature of the one true God. The belief in three separate Gods is not Trinitarianism, but tritheism, which is how the Mormons view the Godhead.
The Trinity is from pagan origins
This is an argument of false cause (i.e., misrepresents the cause of something). In pagan constructs, they worshipped and believed in three separate gods. The Trinity asserts one true eternal God revealed in three distinct inseparable Persons. The Trinity is indigenous only to Christianity. The burden of proof rests squarely on those who make this kind of assertion—asserting something does not prove anything.
“Trinity” is an unbiblical term
This is a very popular objection especially among JWs. For the JWs to argue that the Trinity is not true because the exact word “Trinity” is absent from the Bible is self-refuting. If that kind of reasoning was true, then, the JWs would have to admit: the “Trinity” cannot be pagan, for the word “Trinity is not found in any pagan sources either. Further, if the *non-biblical words = false doctrine* argument were true, then, the Watchtower [WT] must be a false religion for distinctive words that describe their organization are not contained in the Bible either such as “theocracy.” Even the mispronounced and mistransliterated term “Jehovah” was not found until the early thirteenth century, as admitted by the WT.
It is also self-refuting for Oneness advocates to pose the same argument. For many Oneness doctrinal terms that denote the Oneness concept of God are not found in Scripture either (e.g., “manifestations,” “modes,” “offices,” “unipersonal,” “monad,” etc.).
So on one side, both Oneness believers and JWs argue that the “Trinity” cannot be true because the exact term is not contained in Scripture, but on the other side, they both will assert the opposite: non-biblical terms can be used to justify their distinctive doctrines, which they say are biblical. In point of fact, and what is not at all considered, is that terms like, “incarnation,” “self-existent,” etc. are not mentioned in Scripture and both are biblical truths, which by the way, all Oneness believers agree upon.
If we were only limited to strict biblical words, then, when teaching out of the NT, we would have to use only Koinē Greek words that the authors used! Employing extra-biblical terminology does not violate the rules of sola-Scriptura (Scripture alone), as long as the terminology is consistent with Scripture.
In other words, the term “Trinity” is merely a precise doctrinal word that defines the biblical revelation that is so overwhelmingly found in Scripture: There exists only one true God. Scripture also presents that there are three distinct Persons who share the nature of the one true God—the Father, the Son, and the Holy Spirit. Therefore, the church has used the term “Trinity” to describe the biblical data as with “incarnation” (cf. John 1:14) or God’s self-existence (cf. Ps. 90:2) all of which are biblical concepts. Again, this point must be understood: We cannot confuse the biblical data with doctrinal words that define that data.
The Trinity doctrine did not emerge until the 4th century
This is an argument from ignorance. First of all, the term was first used in the East as early as A.D. 180 to describe God by early church apologist, Theophilus, bishop of Antioch. Further, it is completely misleading to say that the doctrine of the Trinity did not emerge until the 4th century. This is a meaningless objection—for it confuses *doctrinal terms* with biblical revelation (as discussed above). The question of what is and what is not biblical is not determined by doctrinal terms, but rather the exegesis of the text. For a term to be “biblical,” it must be substantiated by the clear biblical data—i.e., what is stated in the pages of Scripture. Thus, we are not, as Paul instructs, to “go beyond what is written” (1 Cor. 4:6 NIV).
Scripture presents God as triune. As Christians, we must present a positive affirmation of the gospel (i.e., the true God) and a biblical defense to those who oppose it. For this glorifies God. The JWs spend literally thousands of hours teaching (in literature and personal interaction) against the Trinity. In 2006, they conducted over six million Bible studies every week worldwide! Thus, we must take the time to accurately present the doctrine of the Trinity (the one true God) in our gospel presentation. Pastors especially should be mindful that by never mentioning the Trinity, it is nearly as bad as rejecting the doctrine itself.
 Ontological (by nature) monotheism (one God) is the doctrine that there exists only one God by nature (cf. Deut. 4:35; Jer. 10:10-11). Mormons, although, claim that the Father, Son, and Holy Ghost are “one God,” but only in the sense of “unity,” not one in essence. But, as they assert, these three are three “separate” Gods, with the Father as the head God in whom they worshiped alone—thus, the Mormon view of the Godhead. But whether one or more Gods are worshiped is irreverent, the question is: how many true Gods exist? The fact that the Mormons believe that many “true” Gods exist, therefore, categorizes the Mormon people as overt polytheists (the belief in many true Gods) and hence, non-Christian. Not only in the OT, but in the NT as well, strict monotheism was strongly asserted (e.g., Mark 28:29; John 17:3; 1 Tim. 2:5).
 Simply, the Judaizers taught that one had to practice the OT law, rituals, ordinances, etc. (esp. circumcision), to obtain salvation. And this, was the primary reason as to why Paul wrote to the Galatians.
 The Gnostics (from gnōsis, meaning “knowledge”) held to a dualistic system: spirit was good and all “matter” (esp. flesh) was inherently evil; some even taught that “matter” did not exist; it was illusory—as with the theology of Christian Science today. Both the Apostle John and Paul specifically refuted this teaching (esp. in Col. and 1 & 2 John).
 As seen above.
 E.g., Isa. 9:6; John 1:3; Col. 1:16-17)
 E.g., John 1:1, 18; Titus 2:13; Heb. 1:3, 8-10
 Of course, the OT and NT teaching of “one God” (i.e., monotheism) does indicate or equate “one person” as *unitarian* groups such as Jews, Muslims, JWs, Oneness Pentecostals, etc. presuppose. Monotheism simply means “one God” (viz. “one Being”). To argue that “one God” equals “one person” is to argue in a circle. It assumes what is meant to be proven.
 These examples, however, are not necessarily in order of usage. Further, this is not an exhaustive list, only a sample of some of the main objections that are utilized most commonly by anti-Trinitarians.
 That is, the Watchtower Bible and Tract Society, which is the organization of the JWs.
 The WT publication, Aid To Bible Understanding, states:
The first recorded use of this form [Jehovah] dates from the thirteenth century C.E. Raymundus Martini, a Spanish [Roman Catholic] monk of the Dominican Order, used it in his book Pugeo Fidei of the year 1270 C.E. (Aid To Bible Understanding, 1971, pp. 884-5).
For more information on the term “Jehovah” see www.christiandefense.org/Article on Jehovah.htm
 In logic, this kind of argumentation is called “special pleading” for it “pleads” to or argues only one side of the evidence while ignoring the other side.
 The Father, the Son, and Holy Spirit are called “persons” for the simple fact that (a) they all possess *personal* attributes (e.g., they communicate, make decisions [viz. a will], exercise emotion, etc. also referring to themselves as “I” (egō)—the hallmark of personhood. Even more, Jesus used first person pronouns (“I”) to refer to Himself and third person pronouns (“He,” “His”) to refer to the Father and the Holy Spirit (cf. John chaps. 14-16). Note that anti-Trinitarians such as JWs have no problem seeing the Father as a person, but the same evidence that demonstrates the personhood of the Father can be equally applied to the Son and especially the Holy Spirit. The JW’s are taught that Satan is a person because he communicates, however, that is true of the Holy Spirit at many places (e.g., Acts 10:19; 13:2) and yet they deny the personhood Holy Spirit due to their prior theological commitments: the Trinity is a false doctrine, thus, the Holy Spirit is merely Jehovah’s active non-personal force.
 Cf. Theophilus, To Autolycus, 2.15, in The Ante-Nicene Fathers [ANF], vol. 2. And the term “Trinity” was first used in the West around A.D. 213, by the brilliant church theologian, Tertullian of Carthage (cf. Tertullian, Against Praxeas, 2, in ANF, vol. 3).
 Many falsely assume that the doctrine of the Trinity was not developed until the Council of Nicea (A.D. 325). But the Trinity was not even discussed there.
 E.g., Titus 1:9, 13; 1 Peter 3:15; etc.