The doctrine of Trinity is false
This doctrine is generally accepted by all denominations of Christians today each with their own subtleties. The doctrine of Trinity is based on the creed adopted by the Council of Nicea which took place over 300 years after Christ. There do exist Unitarian churches that reject Trinity and these groups invariably have used the bible to refute the doctrine. Here is a excerpt of the creed produced by the Council of Nicea and a couple of clear proofs used by the Unitarians that the doctrine is false:
Whoever wishes to be saved must, above all, keep the Catholic faith. For unless a person keeps this faith whole and entire he will undoubtedly be lost forever. This is what the Catholic faith teaches: we worship one God in the Trinity and the Trinity in unity. We distinguish among the persons, but we do not divide the substance. For the Father is a distinct person; the Son is a distinct person; and the Holy Spirit is a distinct person. Still the Father and the Son and the Holy Spirit have one divinity, equal glory, and coeternal majesty. What the Father is, the Son is, and the Holy Spirit is. The Father is uncreated, the Son is uncreated, and the Holy Spirit is uncreated. The Father is boundless, the Son is boundless, and the Holy Spirit is boundless. The Father is eternal, the Son is eternal, and the Holy Spirit is eternal. Nevertheless, there are not three eternal beings, but one eternal being. Thus there are not three uncreated beings, nor three boundless beings, but one uncreated being and one boundless being. Likewise, the Father is omnipotent, the Son is omnipotent, and the Holy Spirit is omnipotent. Yet there are not three omnipotent beings, but one omnipotent being. Thus the Father is God, the Son is God, and the Holy Spirit is God. But there are not three gods, but one God. The Father is Lord, the Son is Lord, and the Holy Spirit is Lord. There as not three lords, but one Lord. For according to Christian truth, we must profess that each of the persons individually is God; and according to Christian religion we are forbidden to say that there are three gods or lords. But the entire three persons are coeternal and coequal with one another .So that, as we have said, we worship complete unity in the Trinity and the Trinity in unity. This, then, is what he who wishes to be saved must believe about the Trinity .This is the Catholic faith. Everyone must believe it, firmly and steadfastly; otherwise He cannot be saved. Amen.
One simple argument was presented by Arius is that Jesus cannot be equal to God because Jesus says "My father is greater than I" John 14:28
This alone should be proof of the falsity of the doctrine. But just to show it even more clearly and on many more points see the following link:
Jesus is God made flesh!
The doctrine of original sin is false
Not all Christian sects adopt the doctrine of original sin. It is however widely considered to be part of Christianity since is became official Catholic doctrine from the teachings of Augustine at around 500 years after Christ. The implications of this doctrine have since been watered down by the Catholic church but an error is an error and its lingering acceptance is a clear proof of the corruption of the pure teachings of Jesus which took place in Christianity. I leave the full refutation to a Christian group who relies on the Bible to thoroughly refute this doctrine:
The Myth of Original Sin
Christianity simply doesn't make sense
By the doctrine of original sin, a child is born sinful & therefore punishable. I cannot accept this. All children are born innocent. Does a 3-year-old go to hell? They may be mischievous at times but their intentions are nothing more than to experiment to find out the rules.
Without original sin there is no need for any idea of a great event to remove this burden from human beings. The idea that Jesus (peace be upon him) was crucified to take away the sin of man looses its meaning when sin enters and leaves life in the same way it has always done - through the actions of individuals; there is no need for 'salvation' except through repentance and forgiveness of God (as Adam did, as Abraham did and as all believers until the beginnings of Christianity did and they were not Christians.) ; there is then no reason to accept the divinity of Christ or that his mission was profoundly & essentially different from that of previous prophets.
Original sin makes the whole concept of sin confused in Christianity. How do I know if I did something wrong? I didn't someone else did and I get the blame for it!
Another thing I find confused is the idea of sacrifice. The Christian idea seems to be that "the innocent (such as a lamb) must die to save the sinful. (i.e. Christ must die to save man). This seems to miss the point that sacrifice is where people give up something they value highly. I might sacrifice my time, money etc.
Who sacrificed what in the story of Christ's crucifixion?
Certainly no human being. And how can God sacrifice something. - He owns everything & has absolute power over everything. How can he give up anything?!
This is just one aspect of what to me seems the basic problem with Christianity: If Christ died to atone for our sins then this great act must have changed something about the way to salvation, i.e. that before the act people had a certain route to salvation and that after the act the route to salvation is profoundly different. Indeed this seems to be the claim at the heart of Christianity. But if God fundamentally changes the way he judges people in different times from being harsh to being easier, then this can hardly be justice! On the other hand, if there is no fundamental change in the route to salvation, then why all the fuss? - it doesn't really matter whether Jesus died on the cross or not.