How could one enter into the millennial reign without access to the Kingdom? The millennial kingdom will cover the whole earth, and don't take my word for it, go read King Nebuchadnezzar's vision of the giant statute. In the end, Christ smashes that statute as a meteor and blows away all the debris, and then it becomes a mountain and covers the WHOLE EARTH.
Also, loss of faith happens, everyone has their own backsliding phase, everyone makes mistakes, being saved doesn't make you immune to sin, on the contrary you'll be more aware of your sins and Satan will tempts you harder, but by the power of the Holy Spirit, it can be overcome. "Sin is crouching at your door, it desires to have you, but you must rule over it." That's not just God's admonishment to Cain, but to all of us. Got lost? Follow the Spirit and find direction. Holy Spirit always directs to Christ, the true Christ, through the gospel and testimonies.