The Kingdom of God is already here. You become a member of that kingdom as soon as you become a christian. It doesn't start once you die or receive a glorified body.
Everyone backslides to a certain extent. We can ask for forgiveness, if we are truly repentant.
If we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries. (Hebrews 10:26-27)