One-answer,
You say, 'If God sent us a religion to follow, than it must be perfect and preserved. It must be error free because not only it is important for us while we are alive, it is still important while we are dead and it is also important on the Judgement day. It would be the one thing that will decide all these three I mentioned.'
You are quite right, and God did send us a religion that was error free. It is the religion delivered to Moses and the Israelites, and it is called the LAW. If you want an error free religion then follow the law of Moses.
Shall I stop there and allow you to convert to Judaism?
Islam is really a rival (Ismaelite) version of God's law. It sets forth a different set of laws, and a new set of punishments.
Jesus was a Jew, so he had his religion. He never said he was changing his religion. He did not come to abolish the law. He was circumcised on the eighth day, he returned to Jerusalem three times each year to celebrate the pilgrim festivals, he attended the synagogue in Nazareth. He did all that the law of God required of him. He fulfilled the law.
So did he want to start a new religion? No.
But you now want to tell me that the Law of Moses couldn't have been a perfect religion because the people were not able to please God or follow the Law.
Exactly, THE PEOPLE COULD NOT FOLLOW THE LAW, even though the law was perfect. And like the Jewish nation, the Muslim brotherhood also fall short of pleasing God.
God wants a holy people to be his people. This is something religion does not provide. Do you know why? It's because we are all sinners, and fall short of the glory of God.
Have you never broken any of the laws of your religion? Have you never missed a prayer, have you never committed a sin?
ONLY JESUS CHRIST IS WITHOUT SIN. Only Jesus is able to be the 'lamb of God', the perfect sacrifice for sin. No one else was good enough to please God - not even the prophets. None of the prophets were sinless.
To be without sin is to be the spirit of God.
Christ Jesus IS the spirit, or Word, of God.
Redemptionsong
I am a follower of Moses, Jesus and Muhammad peace be upon them all. Jesus peace be upon him didn't come with a new religion neither Muhammad peace be upon him did. As I explained, these were prophets of God who were sent by Him to share the message of worshiping God and only God. Quraan is the last revelation in that aspect.
Jesus peace be upon him said I didn't come to destroy the laws of the prophets, I came to fulfill it. Jesus peace be upon him also talked about the coming of a next prophet.
Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.
8 And when he is come, he will reprove the world of sin, and of righteousness, and of judgment:
9 Of sin, because they believe not on me;
10 Of righteousness, because I go to my Father, and ye see me no more;
11 Of judgment, because the prince of this world is judged.
12 I have yet many things to say unto you, but ye cannot bear them now.
13 Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come.
14 He shall glorify me: for he shall receive of mine, and shall shew it unto you.
15 All things that the Father hath are mine: therefore said I, that he shall take of mine, and shall shew it unto you.
16 A little while, and ye shall not see me: and again, a little while, and ye shall see me, because I go to the Father.
It is not the law's fault that people didn't follow it. It is human's fault because their desires and love for this world is the way they chose. People were given free will and God showed us the way that we must follow in order to be strong and by that way God will help us in keeping his commandments.
We all had sins even the holiest man among us did. But God is merciful and he gave us the chance to repent.
I am sure you are familiar with these terms. I am not inventing them, they are in Islam and they are the teaching of Jesus peace be upon him too.
Being a muslim doesn't automatically make you holy. But you have to seek truth with your heart and the truth shall free you. These are the words of Jesus Christ peace be upon him.
We are not all sinners. We are born to obey God in nature. But it is we who change and alter that. We are born as believers. That is why if a child died who didn't reach the age of being accountable for his own actions he would enter heaven. Because he is a believer in nature. He is not a sinner.The fact that people sin and choose to sin doesn't mean a person is a sinner. This is what I am 100 % against in the teachings of Christianity.
18 I will arise and go to my father, and will say unto him, Father, I have sinned against heaven, and before thee,
19 And am no more worthy to be called thy son: make me as one of thy hired servants.
20 And he arose, and came to his father. But when he was yet a great way off, his father saw him, and had compassion, and ran, and fell on his neck, and kissed him.
21 And the son said unto him, Father, I have sinned against heaven, and in thy sight, and am no more worthy to be called thy son.
22 But the father said to his servants, Bring forth the best robe, and put it on him; and put a ring on his hand, and shoes on his feet:
23 And bring hither the fatted calf, and kill it; and let us eat, and be merry:
24 For this my son was dead, and is alive again; he was lost, and is found. And they began to be merry.
25 Now his elder son was in the field: and as he came and drew nigh to the house, he heard musick and dancing.
26 And he called one of the servants, and asked what these things meant.
27 And he said unto him, Thy brother is come; and thy father hath killed the fatted calf, because he hath received him safe and sound.
28 And he was angry, and would not go in: therefore came his father out, and intreated him.
29 And he answering said to his father, Lo, these many years do I serve thee, neither transgressed I at any time thy commandment: and yet thou never gavest me a kid, that I might make merry with my friends:
30 But as soon as this thy son was come, which hath devoured thy living with harlots, thou hast killed for him the fatted calf.
31 And he said unto him, Son, thou art ever with me, and all that I have is thine.
32 It was meet that we should make merry, and be glad: for this thy brother was dead, and is alive again; and was lost, and is found.
What is this from the bible? It is how a person sin and how he repents. It is as if he is dead and than came to life.
Your suggestion that God wants us holy suggests that God would hold against us how he created us because you said humans are sinners. Do you really think that?
We have all sinned and we should repent. God accepts repentance from anyone if it is true for He know the intentions, He is the All Knowing.
We will be held accountable for our actions. We will be judged by our actions and faith side by side. It is not only faith alone, neither it is works alone. If that would have been the case God would replace our mind with more faith.
Besides do you think it is fair that only people who believe that Jesus peace be upon him died for their sins would enter heaven? Isn't it like a free ticket to heaven?
Please read this from the bible
Matthew 7
21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
And all real evidence post to a warrior plagiarizing previous mythology. 
