1. If the Bible teaches that there is one God, why do Catholics teach that that the Father, the Son, and the Holy Spirit are all God? That’s three Gods.
Perhaps the most common objection to the Trinity is that the numbers don’t seem to add up. Those who don’t accept the idea of a Trinity are usually under the impression that Catholics are content to say that the difficulty can be resolved by taking refuge in the "God is a mystery" argument.
Believing in a mystery does not mean believing in something that is unreasonable or illogical but rather in something you do not have the capacity to ever know completely—such as God. It isn’t something you can’t know anything about; it is something you can’t know everything about.
You’ll still need to demonstrate that the idea of three Persons and one God is not illogical. To begin, explain the difference between being and person. Beinganswers the question "What?" and person answers the question "Who?" For example, pointing to my mother, the question "What is she?" is answered with, "A human being." The question of "Who is she?" is answered with, "Janie."
All persons are beings, but not all beings are persons. For example, you are one being and one person. But a dog is one being and zero persons. With regard to the Trinity, there is one being, which is God, yet there are there Persons: Father, Son, and Holy Spirit. This is not illogical. If one were to say that there is one God and three Gods, or one Person and three Persons—that would be illogical. But one Being and three Persons is not a contradiction.
2. Some people say that Trinity was one divine Person using three different modes. Is that sort of what Catholics teach?
This is an ancient heresy that has resurfaced in recent years in the Oneness Pentecostal denomination, so you’ll need to be on the lookout for it in your discussions. Originally, this idea was known as modalism, sabellianism, or patripassianism. The Church rejected the concept, since Scripture is clear in demonstrating that the three Persons of the Trinity are distinct. For example, when Jesus prayed to the Father in the Garden of Gethsemane, it is clear that he was not talking to himself. When Jesus told the apostles that he was leaving them to go to the Father in order to send the Holy Spirit, it becomes very confusing if there was only one person playing three roles. If Jesus was the Father, why would he have to go somewhere to be with himself, and if Jesus is the Spirit, why would he have to leave in order to send himself?
3. If the Father, Son, and Holy Spirit are all God, why didn’t the Jews know that? Their great prayer was, "Hear O Israel, the Lord thy God is one God!"
There are two things to look at here: Why didn’t the Jews know of the Trinity, and where is there a Triune God in the Old Testament?
Jesus did not stand in the manger and announce his divinity, and neither did God stand on Mount Sinai and give Moses a theological exposition of the three distinct Persons who are one in being and nature, existing consubstantially and eternally. The Lord is like a good teacher who reveals his truth by slowly planting seeds and then evoking the truth of the conclusions from his students. His revelation to the Jews was a gradual process, and so there is no problem if he wished to reveal more of himself to humanity as time progressed. As Jesus said, "I have many things yet to say to you, but you are not able to bear them at present" (John 16:12).
Similarly, Jesus revealed that his kingdom would be like a mustard seed that grows into the largest of shrubs. There is an organic development in understanding that takes place over time. This is not a mutation or contradiction of a prior understanding, but the natural flowering of truth.
Where are the Old Testament seeds of the Trinity? In Deuteronomy 6:4, one finds the Shema, the Jewish expression of monotheism: "Hear, O Israel: The LORD our God is one LORD." There are two words in Hebrew for one. Yachid means only one. Echod means a compound unity or a united one—as in Genesis: "evening and morning . . . one day" or "husband and wife . . . one flesh." This second word, echod, which describes a unity of beings, is the one used to speak of God, who is not by essence a solitude, but a unity of three persons—Father, Son, and Holy Spirit.
Other implicit references to the Trinity in the Old Testament include Gen. 1:26, which reads, "Let us make man in our image, according to our likeness." Since God was alone at the time of creation (Isa. 44:24, Neh. 9:6) with no other gods, this expression of plurality must refer to God himself. At the Tower of Babel God says, "Let us go down," yet no one else comes down with him (Gen. 11:7).
Still, the Jews did not have an understanding of the Trinity. It was only in Christ that God the Father revealed he had a Son, and that this Son was to send the Spirit. Monotheism was unheard of at the time of ancient Israel, and if Yahweh tried to announce that he was one God in three Persons, it would have seemed to be tritheism (worshiping three gods) to the polytheistic people of the day. God waited for the Christian era to reveal his true nature.
4. I have no problem believing in God the Father, but where does Jesus claim to be God?
Who created the world, forgives sins, gives eternal life, answers prayers, and is worthy of worship? Few would argue that anyone but God is capable of these things. But the Bible teaches that Jesus does them all (Heb. 1:10; Matt: 9:6; John 10:28; John 14:13; Rev. 5:13–14, respectively).
In John 8:58, Jesus takes this sacred name of God (cf. Ex. 3:14), and applies it to himself: "Amen, amen, I say to you, before Abraham came to be, I AM." Only God may use this title of himself without b.aspheming (Ex. 20:7, Deut. 5:11), and the punishment for misusing his name is death by stoning (Lev. 24:16). Thus Jesus’ good Jewish audience immediately recognized the sacred name, and as a result they picked up stones to kill him when he applied that name to himself (John 8:59).
John 1:1 reads, "In the beginning was the Word, and the Word was with God, and the Word was God" (emphasis added). John begins his Gospel by proclaiming the divinity of Christ. Aquinas noted in his Sermon-Conferences on the Apostles Creed that John 1:1 refutes three major Christological heresies in one swipe. By saying, "In the beginning was the Word," the heresy of Photinus (Jesus was a created being) is destroyed. When John says, "and the Word was with God" he refutes the heresy of Sabellius (Jesus was the Father). Lastly, when John says that the "Word was God," the Arian heresy (Jesus was not God) also collapses.
Another compelling verse to consider is John 20:28, where the apostle Thomas says to Jesus, "My Lord and my God." In the Greek, this sentence reads literally, "The Lord of me and the God of me." It would be nothing short of b.asphemy for Jesus not to rebuke Thomas if he were wrong. Jesus does nothing of the sort, but in fact he accepts Thomas’ profession of his identity as God in the next verse.
5. What evidence is there that the Holy Spirit is equal to God?
Those who deny the Trinity usually do not think that the Holy Spirit is a Person, let alone God. For this reason you will probably have to deal with both issues.
Begin with the personhood of the Spirit. In Acts 13:2, the Holy Spirit says, "Set Barnabas and Saul apart for me for the work to which I have called them" (emphasis added). This is a clear reference to the Holy Spirit as a personal being, not some kind of impersonal force.
John 16 refers to the Holy Spirit as a "he" ten times in a single chapter. Scripture also reveals that the Holy Spirit can be lied to (Acts 5:3), can speak (Acts 8:29; 10:19, 20; 13:2), hear (John 16:13–15), teach (John 14:26), reprove (John 16:8-11), pray and intercede (Rom. 8:26), love (Rom. 15:30), be grieved (Eph. 4:30), and be blasphemed (Mark 3:29). Only a person is capable of these attributes and abilities, and only God can be b.asphemed.
Some argue that since the Holy Spirit is a Spirit, he is not a person. But God is a spirit. Satan is a spirit. Angels and demons are spirits—and all of these are persons. The classic definition of a person is an individual substance of a rational nature. The Holy Spirit is obviously an individual substance, and the above verses confirm that he is of a rational nature.
This should demonstrate sufficiently that the Holy Spirit is a personal being, and so now one must prove that he is God. Any number of biblical passages can be used to support this. Acts 5:1–4 explains that a lie to the Holy Spirit is a lie to God himself. (To see verses where the Holy Spirit is equal to God, compare Isa. 44:24 and Mal. 2:10 with Job 33:4, and Ps. 104:30. Also worth comparing is Ex. 17:2 with Heb. 3:9 and Jer. 31:33 with Heb 10:15–16.)
The Bible contains a number of other passages where the Holy Spirit is on par with God. For example, the Holy Spirit is everlasting (Heb. 9:14), all knowing (1 Cor. 2:10) and omnipresent (Ps. 139:7)—attributes that only God has. Only the Catholic understanding of the Trinity can reconcile these passages.