"But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith," (Gal. 5:22)
Christ providing eternal life for all those who believe (by the cross) was promised before the world began, it was the word of God, His eternal purpose. However, man did not have access to that eternal purpose until Christ came to earth and obeyed the word being faithful to the promise of the Godhead.
 Though he were a Son, yet learned he obedience by the things which he suffered;
 And being made perfect, he became the author of eternal salvation unto all them that obey him;
Christ was faithful unto death so that we would not have to be. Once we put our faith in Christ, His faith is what keeps us justified before God since we are members of His body, even if we were to stop believing.