Jesus says this hours before his arrest, to men who are about to watch everything they have built their lives on collapse. He has just told them that one of them will betray him, that Peter will deny him, and that he is going where they cannot follow. Then: Let not your heart be troubled.
The reason he gives is not that things will be fine. It is a person and a place. Ye believe in God, believe also in me. In my Father's house are many mansions. He grounds their steadiness in a destination they cannot see and a relationship they can.
Notice how ordinary and domestic the comfort is. Not a doctrine — a house, with room in it, prepared by someone who knows them. I go to prepare a place for you. The care is specific. It is not a general provision for the saved; it is a place, for you.
And the promise is not that they will find their own way there: I will come again, and receive you unto myself. He does not leave them with a map. He leaves them with the assurance that he will come back for them personally.
Thomas admits he does not understand the way, which is a relief to anyone who has ever felt the same. Jesus does not scold him. He answers, I am the way.