Jesus will be sent down to the Earth once again at the end of the world and people will realise what they have done. In fact christians will realise their mistake the minute they die, on the day of judgement they will be questioned and will answer that Jesus said himself that he was the 'Son' of God. Isa will deny doing so and therefore you will recieve severe punishment.

How can Jesus be God when he denies being absolutely good, and says that only God is good (cf. Mark 10:18)?

By wrenching such statements out of their immediate and overall contexts, the impression is given that the Lord Jesus is actually denying he is God since he denies that he is good in the sense that God is good. The questioner's logic goes something like this:

God alone is (absolutely) good.
Jesus is not (absolutely) good.
Therefore, Jesus is not God.

The Jesus of the Bible is not the "Isa" of the Koran. The Jesus of the Bible said, "God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.  For God did not send his Son into the world to condemn the world, but to save the world through him. Whoever believes in him is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God’s one and only Son." John 3:16-18

So believe it or be condemned on the judgment day - your choice.

John 3:36  "Whoever believes in the Son has eternal life, but whoever rejects the Son will not see life, for God’s wrath remains on him."