It is unfair to label Thomas as the doubter.  His doubt is no greater than that of the other disciples or us.  Jesus responds to honest doubt gently and with a call to come and see - check it out.  He is more true than anything in this earth so he is not afraid of honest inquiry.  However, doubt can also arise from hardness of heart, where we really don't want to know the truth.  That doubt is dangerous spiritually.  Doubt is not sin but it is the basis for sin.  The antidote for doubt is to dwell on the character of God, his truth, faithfulness, changelessness.