Salvation is fully and finally accomplished when you believe and trust in Jesus. The death, burial and resurrection of Jesus Christ paid the Blood Atonement for the sins of the world. It is a free gift, yet came at an unimaginable cost to Him and to the Father.
What is the basis for the Church? It is a Spiritual Kingdom confined to earth for a period of time until removed in the Rapture and Israel stands as Priests of the Earth.
Those who watch for the Rapture have authority from Scripture.