The Suggestion spell influences a creature by suggesting a course of action:
You suggest a course of activity (limited to a sentence or two) and
magically influence a creature you can see within range that can hear
and understand you. Creatures that can’t be charmed are immune to this
effect. The suggestion must be worded in such a manner as to make the
course of action sound reasonable. Asking the creature to stab
itself, throw itself onto a spear, immolate itself, or do some other
obviously harmful act ends the spell.The target must make a Wisdom saving throw. On a failed save, it
pursues the course of action you described to the best of its ability.
The suggested course of action can continue for the entire duration.
If the suggested activity can be completed in a shorter time, the
spell ends when the subject finishes what it was asked to do.You can also specify conditions that will trigger a special activity
during the duration. For example, you might suggest that a knight give
her warhorse to the first beggar she meets. If the condition isn’t met
before the spell expires, the activity isn’t performed.If you or any of your companions damage the target, the spell ends.
The description suggests the idea that this course of action sounds reasonable to the target, in such a way that it could be a decision that the target took on his own. Furthermore, contrary to other illusion spells (e.g. Friends), the target won't notice that he has been charmed.
However, I am not sure whether the ideas suggested by the spell persist after the spell ends. Consider the following example:
A creature owns a magical item connected to evil forces. He is using that item for his purposes. Another creature casts Suggestion by saying: "You feel that this item is ruining your soul. You will get rid of it, and you will go praying for 2 hours in order to purify your soul."
Supposing that the target considers it reasonable, and gets rid of the item. Then, for some reason, the spell ends. Now, how will the target react? Will he keep considering that idea reasonable, or will he try to get the item back?
Do the ideas that justify the Suggestion spell persist after the end of the spell?
Best Answer
No, twice over.
Believing something is not a course of action and suggestion is incapable of implanting beliefs. Suggestion can't suggest mental conditions or states, only activities.
Even if it could, everything that suggestion made someone do ends when the duration ends. For beliefs what that looks like would be kind of strange (wouldn't even a temporary belief continue to have effects on your mind?), which is fine, because suggestion can't induce beliefs.
So no part of this, including probably the actions, would work:
The first sentence won't work because it's not an activity. The subject would hear you say “You feel that this item is ruining your soul,” and would react just like anyone would who were just told how they feel — disbelief and mistrust.
The second sentence would probably not work, because someone with an awesome magic item would not find “get rid of it” to be reasonable, and someone who doesn't feel like their soul is in danger wouldn't find “go pray for two hours to purify your soul” reasonable.
If you want to pull off suggestions that rely on beliefs that the target doesn't already have, you have to do it the old-fashioned way: persuade them to believe something first.