God's first promise was a Savior, one who would pay the price for our sins. All men were dead in their sins, and there had to be a way made to cleanse us so God could give us the second promise, which was Himself--the Spirit. Man was made for God (Colossians 1:16). Both promises are God giving Himself for man because of who He is: Love (I John 4:16), Christ came and paid the price to redeem us back to God. The Spirit was given to make us sons, not by a legal transaction, but by giving us life and making our relationship a reality.
So we can see that Jesus Christ came and paid the price, dealt with sin that we might once again know God's indwelling in this body of clay. Our bodies are made to be the temple of the Holy Ghost, which is clearly seen throughout the New Testament. Of what value is a temple, unless the Holy Ghost comes to indwell? There is no other salvation or way of life that God has made, other than the believer becoming a partaker of the divine nature of God. Peter tells us that we have been given "all things that pertain unto life and godliness, through the knowledge of Him that hath called us to glory and virtue" (II Peter 1:3).
It's not enough to receive; then we must continue to walk in the Spirit. Many today have experienced something along the way, but they have failed to know the Lord and to grow and mature as God has designed. Let us not be children, but let us by faith enter into what God has prepared for us. He tells us that the anointing will teach us, and bring us into the fullness of all that He is. We must come back to the simplicity of this walk of faith, responding in obedience to a loving Father. We are created with a choice, and God holds man responsible. We cannot save ourselves--that is by God's grace. But we are responsible for what we do with that grace, and God's free gift of eternal life. We are responsible to abide (John 15:4-6). This can only be done by faith, in an ongoing relationship.
In Ephesians 4:5, Paul tells us that there is one baptism. This is not water baptism; it is the baptism of the Spirit. The beginning of life is when we are baptized in the Spirit. We then go on to know the fullness of the Spirit as we submit to Him in our union with Him. To know the fullness, there must be a continuous hungering and thirsting, a continuous partaking of His life.