The truth is that the trinity is absolutely a mainstream Christian teaching.
It is taught by Christian churches, clergy, theologians and mainstream denominations any group which does not accept the Trinity - eg Jehovah's Witnesses, Mormons - are cults'
The trinity is taught in the Nicene creed which describes Jesus as God from God, light from light, true God from true God, begotten NOT made. The rules of this forum say that if you want to call yourself a Christian; in the bit in the personal profile which says "faith" - you have to affirm the Nicene Creed. Therefore, the trinity is upheld on these Christian forums as well.
IMO, the trinity is central to our faith; every part of the trinity is involved in our salvation, and without the trinity I do not believe we could be saved, born again, Spirit filled Christians; children of God and heirs with Christ.
God the Father created the world through Jesus and in the power of the Spirit. Mankind rebelled, and broke the relationship that they had with their heavenly Father. There was, and is, no way of restoring that relationship - on our part. God gave the sacrificial system, but the Jews were still unable to keep the covenant; 1000s of animals were being sacrificed and people were no closer to God. So God came to earth himself, in the person of Jesus. God lived among us, (John 1:14), walked in our shoes, felt our pain and experienced everything we experience. Jesus was like us in every way, except that he did not sin, Hebrews 4:15. Jesus was the Lamb of God who takes away the sin of the world, John 1:29, and our Good Shepherd who lay down his life for our sins, John 10:11. The angel told Joseph, even before Jesus' birth, that he would save his people from their sin, Matthew 1:21, and Jesus said that this was why he had come, Mark 10:45.
Before he died, Jesus told his disciples that if he did not go away he would not be able to send the Spirit - the Holy Spirit - to them, John 16:7. After his resurrection he told them to wait until the Spirit was sent to them, and this happened on the day of Pentecost, Acts 2. The Spirit was poured out on the believers that day, and that was a fulfilment of a prophecy made by Joel, Joel 2:28. When he ascended, Jesus became our mediator, Hebrews 9:15. He took our humanity into heaven . So if people had once believed that God was remote from his creation, that he made us then sat on a cloud and left us to get on with it; they need believe it no longer. Jesus is the perfect mediator between man and God because he was both - the world was made through him and then he himself came to be part of that world.
The prophet Ezekiel also said that God would, one day, put his Spirit IN people.
Jesus said that we cannot be born again, have new life and enter the kingdom of heaven unless we are born of the Spirit, John 3:3-5. Paul says that it is the Holy Spirit who assures us that we are children of God, Romans 8:16, so unless we have the Holy Spirit, we cannot call God, Father. He also says that we cannot declare Jesus as Lord without the Spirit.
If we have the Holy Spirit, the Spirit of God, living IN us, he is also our assurance that we have eternal life and will one day be with Jesus and receive our inheritance, 2 Corinthians 5:5.
When we become Christians, we say that we give ourselves completely to him, but he has also given himself completely to us, in a way we can't even begin to imagine.

If Jesus wasn't God when he was on earth, then it was just a man who died on the cross - and I don't believe that any man, however highly anointed, could have the ability or authority to give us eternal life and reconcile us to the Father. That being the case, we would surely still be dead in our sins. So how could we be saved, or know that we are saved?
If Jesus wasn't God then it means that God is remote from his creation, unattainable. We would need to convert to Judaism and offer 1000s of animal sacrifices - which even they don't offer any more - to be one of God's people and under his covenant, and even so, the best that we could hope for was forgiveness, making some kind of atonement for our sin - NOT reconciliation to our Maker; the right to call ourselves children of God, heirs, together with Jesus, to the king of heaven. We have a NEW spiritual identity; we have the right to say that we are children of the King, which is SO important to remember when the devil tries to trip us up and tells us we are worthless, sinners and nobodies. This is not any of our doing, we don't say this because of OUR faith or anything WE have done - it is all Jesus. Paul says that we have every spiritual blessing through Christ - if we believe and accept him.

The whole Gospel is that we were dead in our sins with no hope of reconciliation with God, no way of earning his love and no hope of heaven. The wages of sin is death, Romans 6:23 - eternal and complete separation from God. That is the price we would have had to pay for our sin, and totally what we deserved. But because God loves us so much, he did not want his creation to suffer like that and never know him, so he paid that price himself, in Jesus. I know that raises the question, "did God die?" or "how can God die?" and the answer is, I have no idea. But what's the alternative; that either a man or an angel died on the cross? How then do we have a Gospel - how is that Good News for US?
If the Holy Spirit, the Spirit of God, is not God, then we do not have GOD living in us. Which means, firstly that Jesus, the apostles and Paul were all lying, and secondly, that the best we can hope for in this life is that we do our best, try to be holy and please God and hope that somehow he will bend his own rules and allow us into heaven anyway. If the Holy Spirit is not God, then what are the gifts he gives and the fruit he produces; who are they from?
And if the Father, Jesus and the Spirit are all God, and there is only ONE God - one Father, one Son and one Spirit, who are all divine - how can that be, UNLESS they are one?
With respect, I suspect you recommend them because you wrote them.
How about quoting some well respected Christian theologians and commentators - maybe like John Stott, William Barclay, Tom Wright? If these, and others, backed up what you are saying and interpreted the Scriptures you quote in the way that you do, I would imagine that far more people would be willing to listen. These are well read, well trusted and accredited authors and theologians - it would surely help your argument no end if you could say, "John Stott, Billy Graham (or whoever) agree with me and teach this about Scripture."
But I think the reality probably is that all the while you say, "this is what this Scripture means .... and this is a quote from my own blog which proves it", you'll find that many of us disagree.