Ephesians 1:13-14 says, In Him (we) also trusted, after (we) heard the word of truth, the gospel of (our) salvation; in whom also, having believed, (we) were sealed with the Holy Spirit of promise, Who
is the guarantee of our inheritance until the redemption of the
purchased possession, to the praise of His glory.
Our inheritance is eternal life. There is no eternal life
without spiritual life which is fellowship with God. He made us, who were DEAD
in sin, alive in Him, Christ. That life is forever. A spiritual
life of fellowship with God is guaranteed. What is not guaranteed
is our spiritual perfection in Him... our growing into the mature
man, to the measure (standard of judgment; determined extent, which is
lofty) of the stature of the fullness of Christ; which is our spiritual
completeness in Him. It’s not guaranteed because that part is up
to us, and apparently there are some believers in the world who either don’t
know that we have to grow into it (ignorance of the word),
or have decided it’s not necessary (complacency).