The question that Thomas should have asked was the reason why Jesus would return to His Father.  The reason He went back to Heaven makes Heaven truly attainable for anyone who truly wants to go there.  The sequence of events in Holy Week appeared to the disciples as a horrible catastrophe.  Jesus had got Himself crucified!!  Their whole world had come crashing down about their ears.  From God the Father’s viewpoint it was a great success, a plan accomplished.  God sent Jesus to die on the cross for you and me.  The Bible says - But when the time had fully come, God sent forth his Son, born of woman, born under the law,to redeem those who were under the law, so that we might receive adoption as sons. Gal 4:4-5 (RSV)  It also says - For God sent Christ Jesus to take the punishment for our sins and to end all God's anger against us. He used Christ's blood and our faith as the means of saving us from his wrath. In this way he was being entirely fair, even though he did not punish those who sinned in former times. For he was looking forward to the time when Christ would come and take away those sins. Romans 3:25 (Living)  

Some of this is all very technical.  So let me try to simplify the order of events.  Jesus left Heaven and was born at Bethlehem.  He lived for 33 years on the earth.  During this time He was tempted but did not sin.  He was arrested and died on the cross.  Whilst He was on the cross God judged the sum totality of human sin.  Because Jesus is a man he can die in the place of one man.  Because He is God the Son he can die in the place of every human who has ever lived and who will ever live.  He went back to Heaven to present to the Father the fruits of His