First ,who told you Not to open the door? Is that love or an act of kindness?
Jesus said that true christians will be recognized by their fruit, their love for one another. (John 13:35)By this all will know that YOU are my disciples, if YOU have love among yourselves
Who ever told you that; which do you think will be showing an act of kindness what that person says or what Jesus said?
IN these times we are living in it's true that we have to be very careful to whomever we open the door to, because a person who say's they are, may not be. So be careful about that anyway. However here is a good scripture to consider when someone comes to your door stating that they are a witness.
in your own bible.
Matthew 10:11“Into whatever city or village YOU enter, search out who in it is deserving, and stay there until YOU leave. 12 When YOU are entering into the house, greet the household; 13 and if the house is deserving, let the peace YOU wish it come upon it; but if it is not deserving, let the peace from YOU return upon YOU. 14 Wherever anyone does not take YOU in or listen to YOUR words, on going out of that house or that city shake the dust off YOUR feet. 15 Truly I say to YOU, It will be more endurable for the land of Sod′om and Go·mor′rah on Judgment Day than for that city
So consider, what group of people all over the world are going from house to house which is also fulfilling that scripture, and also consider what the what the witnesses are trying to do.
We are just trying to find discerning ones concerning the times we are living in. We take acceptance as well as rejection, though we prefer the fore. And as far as you opening the door let the person who told you that, not to open the door, but as for you, You can make up your own mind as to whether you will or not.
We are people just like you, If you see a little old lady, or child i would say its ok to open the door, if its a single person personating in a suit by themselves, ask to see their literature, the bible or talk through a window if you are concerned about safety, but for the most part you would be fine for true witnesses are harmless and are trying to love our neighbor as Jesus commanded at Mark 12:31.