Since the Fall of Adam, Man has always had the ability to choose the wrong. The easiest way to damnation is to do nothing. Choosing the wrong is our default setting. Following the natural man, following after our appetites and passions, and doing whatever we want, leads to slavery and death.
Accepting Christ alone will save you from Hell. But obedience, good works, and righteousness will determine your reward/kingdom/crown/glory/mansion/calling.
On this Earth we learn three lessons: 1. Choose God over Satan, 2. Master the body (keep the commandments), 3. Submit to God's Church and priesthood authority. Those who choose Satan over God (satanists) will receive no reward and inherit "the blackness of darkness forever". Those who choose God but never learn to master the body (hedonists), will inherit the " least" kingdom of God known as the Telestial reward/glory/mansion etc (glory of the stars). Those who keep the commandments are the honorable people of the Earth will receive a higher Terrestrial glory/kingdom/mansion (glory of the moon). However, to receive the highest Celestial reward (glory of the sun), a person must submit to Christ's priesthood authority. Only the valiant in Christ (labor in the vineyard 1hr or all day) who eventually learn to devote themselves to God's will, will receive the highest reward
The most popular religion in the world is the "god is in my heart" religion. Truly God can and does dwell in the heart. But the Living God has organized His kingdom and Church on the Earth with a living prophet and a local bishop. And those authorities will ask you to do many things and make many sacrifices in the name of Christ. Many people prefer the 'god in their heart" religion that likely doesn't make as many requests of them.
I think Creedal Christians and Latter-day Saints talk past one another. Yes, accept Christ and you're saved. But works are involved and follow in the process of "perfecting the Saints" or "perfecting those who are justified and sanctified in Christ". The LDS works (repentance, ordinances, Christian service) are necessary to receive the highest reward/mansion/glory in the Kingdom of God.
Jesus said unto him, If thou wilt be perfect, go and sell that thou hast, and give to the poor, and thou shalt have treasure in heaven: and come and follow me. (New Testament, Matthew, Matthew 19:21)
I think we confuse the idea of justification with the process of being "perfected in Christ." Christ gave us Apostles and Prophets for the "perfecting the body of Christ", Christ also gave us scripture also for perfection.
That the man of God may be perfect, throughly furnished unto all good works. (New Testament, 2*Timothy, 2 Timothy 3:17)
We must receive all these gifts of Christ's grace. This is how we are saved by Christ's grace "after all we can do." What is "all we can do?" All we can do is "make use of the means the Lord has [already] provided for us".
Or do ye suppose that the Lord will still deliver us, while we sit upon our thrones and do not make use of the means which the Lord has provided for us? (Book of Mormon, Alma, Alma 60:21)