Mosiah 8:17

But a seer can know of things which are past, and also of things which are to come, and by them shall all things be revealed, or, rather, shall secret things be made manifest, and hidden things shall come to light, and things which are not known shall be made known by them, and also things shall be made known by them which otherwise could not be known.

Through all time, God has had prophets, seers, and revelators on the earth to help his children learn from the past, cope with the present, and prepare for the future. As long as there is a man or woman with faith to hear, God has spoken. He who never changes works the same today as in the past--he has prophets, seers, and revelators among us today who encourage us to look to God, have faith, protect our families, keep the commandments, and be of good cheer. If we want to be happy, who knows better than God how to be happy?

Mosiah 8:16

And Ammon said that a seer is a revelator and a prophet also; and a gift which is greater can no man have, except he should possess the power of God, which no man can; yet a man may have great power given him from God.

God does not grant to men all of his power. Some powers are simply not meant for mortals. The power of resurrection, for example, is not given to man. The Lord holds that power for himself. Yet man can be granted power through the priesthood to raise the dead and call them forth from death to return to mortality. Those so raised,  however, are still mortal and will again have their spirits separated from their bodies, to await like all mortals the permanent reuniting of body and spirit that is the God-given gift of resurrection. We as mortals can receive great power over earthly things, but God reserves power over heavenly things.