Mortality is a time of testing.
25 And we will prove them herewith, to see if they will do all things whatsoever the Lord their God shall command them . . .
Elder Bruce R. McConkie wrote:
“Mortality itself is a probationary estate (Alma 42), a time of trial and testing for all men. With reference to His eternal plan to send all the pre-existent hosts to earth, the Lord said: ‘We will prove them herewith, to see if they will do all things whatsoever the Lord their God shall command them’ (Abraham 3:25). But the greatest trials of life are reserved for the saints. They are the ones whom the world hates (Matthew 10:22), and they must overcome the world, if they are to gain the Lord’s approval. They face all that the world faces in the way of mortal difficulties—sickness, disease, calamities, famine, pain, sorrow, death—and in addition their faith in Christ and His work is tested to see if they will serve the Lord at all hazards. ‘I will prove you in all things,’ the Lord says to His saints, ‘whether you will abide in my covenant, even unto death, that you may be found worthy. For if ye will not abide in my covenant ye are not worthy of me’ (D&C 98:14–15). ‘Blessed are they which are persecuted for righteousness’ sake: for theirs is the kingdom of heaven’ (Matthew 5:10).”
(Doctrinal New Testament Commentary, 3 vols. [1965–73], 3:318.)