Have you ever gotten a call telling you someone was coming over or had some one show up unexpectedly at your house and did a quick pick up before the person came in? That happens at our house. Picking things up quickly so the house looks good is not the same as a thorough cleaning.
Jesus said the Pharisees were like that. They cleaned the outside of cups to look good but did not wash the whole cup. They did things to make themselves look good to people but were not clean on the inside. Their religion was a façade.
Jesus warns us that it is not enough to be considered "religious", to look good on the outside. We need to people of integrity, the same inside and out. We can be faithful to God because He is faithful to us.