The fulness of the gospel is that Jesus and our Heavenly Father take up their abode with us during mortality. In that sense the full extent of the good news of the gospel is that our Gods want to reunite with us as soon as possible. They love us and miss us. They want us to be reconciled to them as soon as we will allow. Embracing and receiving the fulness is the single objective of our mortal existence and is supposed to be the focus of all we do as followers of Jesus Christ and members of His Church in these last days.
There are several phrases employed in the scriptures to explain and reinforce this doctrine. To receive the testimony of Jesus is to receive a promise from Him of exaltation while in the flesh, which is also referred to as the Holy Spirit of promise. To receive the testimony of Jesus after mortality secures terrestrial glory. Recipients of this promise (in mortality) become members of the Church of the Firstborn and are worthy of the celestial kingdom. This is also associated with having your calling and election made sure. To be perfectly clear, these things occur when the heavens open up and man literally interacts with the Gods in heaven during mortality. All will interact with them after this life (think final judgement). Spiritual success is piercing the veil and knowing God during this mortal existence. Consider these scriptures:
51 They are they who received the testimony of Jesus, and believed on his name and were baptized after the manner of his burial, being buried in the water in his name, and this according to the commandment which he has given—
53 And who overcome by faith, and are sealed by the Holy Spirit of promise, which the Father sheds forth upon all those who are just and true.
54 They are they who are the church of the Firstborn.
62 These shall dwell in the presence of God and his Christ forever and ever.
70 These are they whose bodies are celestial, whose glory is that of the sun, even the glory of God, the highest of all, whose glory the sun of the firmament is written of as being typical.
71 And again, we saw the terrestrial world, and behold and lo, these are they who are of the terrestrial, whose glory differs from that of the church of the Firstborn who have received the fulness of the Father, even as that of the moon differs from the sun in the firmament.
74 Who received not the testimony of Jesus in the flesh, but afterwards received it.
75 These are they who are honorable men of the earth, who were blinded by the craftiness of men.
76 These are they who receive of his glory, but not of his fulness.
77 These are they who receive of the presence of the Son, but not of the fulness of the Father.
78 Wherefore, they are bodies terrestrial, and not bodies celestial, and differ in glory as the moon differs from the sun.
3 Wherefore, I now send upon you another Comforter, even upon you my friends, that it may abide in your hearts, even the Holy Spirit of promise; which other Comforter is the same that I promised unto my disciples, as is recorded in the testimony of John.
4 This Comforter is the promise which I give unto you of eternal life, even the glory of the celestial kingdom;
5 Which glory is that of the church of the Firstborn, even of God, the holiest of all, through Jesus Christ his Son—
22 For strait is the gate, and narrow the way that leadeth unto the exaltation and continuation of the lives, and few there be that find it, because ye receive me not in the world neither do ye know me.
23 But if ye receive me in the world, then shall ye know me, and shall receive your exaltation; that where I am ye shall be also.
24 This is eternal lives—to know the only wise and true God, and Jesus Christ, whom he hath sent. I am he. Receive ye, therefore, my law.
25 Broad is the gate, and wide the way that leadeth to the deaths;and many there are that go in thereat, because they receive me not, neither do they abide in my law.
“After a person has faith in Christ, repents of his sins, and is baptized for the remission of his sins and receives the Holy Ghost, (by the laying on of hands) … then let him continue to humble himself before God, hungering and thirsting after righteousness, and living by every word of God, and the Lord will soon say unto him, Son, thou shalt be exalted. When the Lord has thoroughly proved him, and finds that the man is determined to serve Him at all hazards, then the man will find his calling and his election made sure, then it will be his privilege to receive the other Comforter, which the Lord hath promised the Saints, as is recorded in the testimony of St. John.” ( Teachings of the Prophet Joseph Smith, p. 150.)” (as cited in General Conference, April 1977, President Romney)
18 I will not leave you comfortless: I will come to you.
21 He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him.
23 Jesus answered and said unto him, If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him.
3 John 14:23—The appearing of the Father and the Son, in that verse, is a personal appearance; and the idea that the Father and the Son dwell in a man’s heart is an old sectarian notion, and is false. (D&C 130:3)
Jesus prophesied that we, the latter-day gentiles, would reject the fulness of His gospel. When we do, He promised He would “bring the fulness of [His] gospel from among [us].”
3 Nephi 16
10 And thus commandeth the Father that I should say unto you: At that day when the Gentiles shall sin against my gospel, and shall reject the fulness of my gospel, and shall be lifted up in the pride of their hearts above all nations, and above all the people of the whole earth, and shall be filled with all manner of lyings, and of deceits, and of mischiefs, and all manner of hypocrisy, and murders, and priestcrafts, and whoredoms, and of secret abominations; and if they shall do all those things, and shall reject the fulness of my gospel, behold, saith the Father, I will bring the fulness of my gospel from among them.
Have we, as a people, lost focus on or belief in this doctrine? Are we fulfilling the Savior’s prophecy by rejecting this ultimate of all doctrines? Once our collective disbelief regarding this ultimate gospel goal becomes pervasive will the Lord remove from us our ability to receive this greatest of all blessings? Has that already started happening? Is the loss of this doctrine and focus the final straw, marking the final step in our march toward being fully ripe for destruction?
I pray to God we might all repent and reclaim this most precious of truths and not sin against His gospel by rejecting the fulness.