Why Believe the Trinity if the Bible Never Mentions It?
Why should anyone believe in the doctrine of the Trinity if the Bible never uses the word "Trinity"? This is a very good question, because anything we believe about God should come from the Bible. Any doctrine that doesn't come from the Bible should be discarded. Many churches, such as the Iglesia Ni Cristo, have doubted the Trinity on these grounds, but if we examine the Bible carefully, we see that the Trinity is actually in the Bible.
We can use words that aren't in the Bible to talk about the Bible.
First, just because a word or phrase is not in the Bible doesn't mean we cannot use it to talk about the teachings of the Bible. For example, the INC has a set of "Fundamental Beliefs". The Bible never mentions "Fundamental Beliefs", but that is no reason to object to the phrase. The phrase is used as a way of summarizing what the INC believes the Bible to teach about important things. Likewise, the fact that the word "Trinity" is not found in the Bible is no reason to object to the word. It is simply a word used to summarize what the Bible teaches about God.
The Trinity is a combination of multiple doctrines.
The reason why the word "Trinity" is not found in the Bible is because the Trinity is not really a single, atomic doctrine. It is the combination of several different doctrines that are taught in scripture. For example, the INC member will have to admit that there is no one verse that teaches the 28 Fundamental Beliefs of the INC. In the same way, there is no one verse that teaches all the tenets of the Trinity. If it is OK to use the phrase "Fundamental Beliefs" as a way of summarizing multiple doctrines, then it is OK to use the word "Trinity" as a way of summarizing multiple doctrines as well.
What does the doctrine of the Trinity teach?
The doctrine of the Trinity teaches that:
1. There is only one God.
2. The Father is God.
3. The Son is God.
4. The Holy Spirit is God.
5. The Father is not the Son.
6. The Son is not the Holy Spirit.
7. The Holy Spirit is not the Father.
We could perhaps list more points, such as the co-eternality and co-equality of the three (Genesis 1:2, Psalm 102:25-27, Hebrews 1:10), but these will suffice for our purposes. If you wish to learn more in depth about the Trinity, this video is very helpful.
The INC can agree that most of these truths are found in scripture. However, The INC does not agree with 3 and 4; that is, that the Son and the Holy Spirit are God.
The Son is God.
The deity of the Son is found all throughout the New Testament, but for the sake of brevity, we will only mention one verse here.
Colossians 2:9 (ESV) -- For in him the whole fullness of deity dwells bodily,
This means that Jesus possesses all the characteristics of God and is himself fully God. It does not mean that God lives inside him, as he does in believers, because the previous verse is lifting Jesus up as the standard and the source of all godliness, not as someone who possesses godliness as a foreign attribute.
The Holy Spirit is God.
The Holy Spirit is not an inactive force, as some claim. He is a person. We are told that he grieves.
Ephesians 4:30 (ESV) -- And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.
We are told he has a mind and that he prays for the saints (those who believe in Jesus).
Romans 8:27 (ESV) -- And he who searches hearts knows what is the mind of the Spirit, because the Spirit intercedes for the saints according to the will of God.
Additionally, we see that the Holy Spirit created; something only God can do.
Job 33:4 (ESV) -- The Spirit of God has made me, and the breath of the Almighty gives me life.
The Trinity teaches there is only one God.
This has already been stated, but the Trinity teaches there is only one God. Therefore, verses that affirm monotheism (that there is only one God) may not be used to attack the Trinity. Trinitarians agree with those verses, and agree there is only one God.
The Trinity teaches that God exists in three persons.
In order for there to be only one God, yet permit the Father, Son, and Spirit to all be God, we must be willing to make a distinction between "being" and "persons". There is only one being of God, though there are three persons. There is only one essence of God, shared by three persons.
Most attacks against the Trinity simply are not willing to allow for these distinctions. In other words, they do not attack the doctrine of the Trinity, but a straw man. The one who attacks the Trinity must be willing to do it on the terms of the doctrine. Few even attempt this, because the doctrine is true, found in scripture, and if one were to attempt to refute it for what it is, they would surely fail.
The word "Trinity" may not appear in the Bible, but everything the doctrine of the Trinity teaches comes from the Bible. We should absolutely affirm the Trinity as a Biblical doctrine. If we do not affirm it, then we do not permit all of scripture to speak. If we do allow all of scripture to speak, then we will see that God is indeed triune.