D&C 76

The Lord describes those that receive the glory of the terrestrial kingdom - these are

" … they who are the spirits of men kept in prison, whom the Son visited, and preached the gospel unto them, that they might be judged according to men in the flesh;
74 Who received not the testimony of Jesus in the flesh, but afterwards received it."
(Doctrine and Covenants Section 76:76)

It continues -

"76 These are they who receive of his glory, but not of his fulness."
(Doctrine and Covenants Section 76:76)

These are they who were indifferent or distracted with the things of this world. That didn’t bother getting a testimony of Jesus in this life.

Amulek pleaded –

"…I beseech of you that ye do not procrastinate …until the end; … behold, if we do not improve our time while in this life, then cometh the night of darkness wherein there can be no labor performed. "(Book of Mormon Alma 34:33)

Receiving a testimony is not easy, it takes work. It requires reading and

Ponder[ing things of God] in your hearts.

Then -

"…ask[ing] God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost." (Book of Mormon Moroni 10:4)

For he promises -

"...I will tell you in your mind and in your heart..." (Doctrine and Covenants Section 8:2)

The testimony of Jesus is not a destination but a process of -

…line upon line, precept upon precept..." (Doctrine and Covenants Section 98:12)

There is also another “so called” homework assignment that also needs to be done while in the flesh to receive full credit – "receive of his fulness"

(Doctrine and Covenants Section 41:5)

"He that receiveth my law and doeth it, the same is my disciple; and he that saith he receiveth it and doeth it not, the same is not my disciple…"(Doctrine and Covenants Section 41:5)

We read of those of the glory of the terrestrial world –

"…these are they who died without law;" (Doctrine and Covenants Section 76:72)

"These are they who receive of his glory, but not of his fulness." (Doctrine and Covenants Section 76:76)

To receive of his fulness, we must diligently seek to receive his law. For it is -

"…the law [that] maketh you free."( Doctrine and Covenants Section 98:8)

In the early days of the Church, the saints were commanded to

"… go to the Ohio; and there I will give unto you my law; and there you shall be endowed with power from on high;" (Doctrine and Covenants Section 38:32)

And then they were commanded -

"Ye shall observe the laws which ye have received and be faithful."
(Doctrine and Covenants Section 42:66) "...And thus ye shall become instructed in the law of my church, and be sanctified by that which ye have received…" (Doctrine and Covenants Section 43:9) "… [for] that which is governed by law is also preserved by law and perfected and sanctified by the same." (Doctrine and Covenants Section 88:34)

21 And they who are not sanctified through the law which I have given unto you, even the law of Christ, must inherit another kingdom, even that of a terrestrial kingdom, or that of a telestial kingdom. (Doctrine and Covenants Section 88:21 - 22)

Though the glory of the Terrestrial Kingdom is a gift - it is not the greatest of all the gifts of God.