So I am Dming a camping that is about the invasion of the shadow creatures to the material plane 5e and we are on the third session and my players are in the elemental forest they just had an encounter with an evil cult that uses possession of other bodies to go to battles so that the actual cultists don't die and as we were playing I made a mistake... I actually gave the chance to my players to be an evil group because the NPC evil Mage offered them a position in their cult. When I thought about that before the session I thought... I will just throw this offer so that they can infiltrate the cult then learn some info about the BBEG and then kill some cultists and escape. Well as you expected they were actually thinking if they want to become an evil group and because I don't like to say to them well that they cannot do that I could use some advice... I usually predict these things but my group has never even thought of playing evil characters ever and I didn't saw that coming. 

Why did the party make this decision? Might they have some sort of carefully calculated plan to infiltrate the cultists, as you predicted, or are they literally just joining the bad guys for the sake of joining the bad guys?

I know you said that you hate telling the players what they can and can’t do (that’s fine! I do too), but to me, this seems like you need to discuss this with your players. Make sure they know exactly what they’re doing, and make sure that everyone wants to play in an evil game and that you, as the DM, want to run an evil game before you start an evil game. (And my guess is that you don’t want to run an evil game.)
Also, joining the bad guys for the sake of joining the bad guys seems like kind of an evil thing to do. If this is a campaign where everyone plays by their character’s morals, there should be at least a few PCs who aren’t fine with this decision.

I’m fairly new to the game, so I might be wrong about how to handle this. Good luck.

Thanks for the help indeed I don't want to run such I campaign because I have already planned half the campaign soo yeah I will discuss it with them so that we can find something that makes us all happy. Maybe an evil oneshot could work I don't know thanks for the tip 

I see a couple ways this could go. First, maybe they step back from the edge and don’t go evil, problem solved.

Second, they choose to, and you go with it. Let them be cultists; it would be a pretty unusual campaign, could be fun for a bit. And after a while, when they see the kind of evil the cultists do, give them a chance to betray the cult and have a redemption arc. 

Third, you talk with them out of character and just say “I never expected you guys to choose to go evil. I’m not at all prepared for this, and, frankly, don’t want to run an evil campaign. Please don’t do it.”

Yep you are right I should just go along with it for a while and if they end up joining they will have some fun for a bit but. You are right I know my players and they wouldn't last more than two sessions of being evil. Thanks for the tip I just came up with the idea of slowly Introducing an evil NPC that I had no idea how to make them talk to.

What alignment where the players to begin with? What class are they? 

A person does not just become evil because of one choice or act, the rules on alignment can be a little bit harsh, it is one of the mechanics I actually dislike because it makes players think in a very black and white way. In real life someone who is Lawful Good is perfectly capable of committing an evil act for a good reason and remaining a good lawful person, alternatively  an evil person can do a really good thing, but still be evil. 

So i would say simply joining the cult does not make them evil yet, it probably does not even shift them from good to neutral. Once they are in if they start doing evil things then there may well be a shift in alignment but again it needs to be a combination of a number of different consistently evil acts that makes this shift occur. 

I would say talk to the players, is this something that goes against the core tennants of any of their characters, and then really push the evil down their throat if they insists on sticking it out. What do they do when a baby is placed on an alter and one of them is asked to sacrifice it? What do they do when asked to massacre a mother and her 3 children, what happens when the cult leader decides the party must sacrifice one of themselves in order to show their dedication to this new cause, or remove a left eye, maybe each of there parents are lined up before them and they are told to pick one to sacrifice? It is amazing how quickly, when faced with the reality of the situation they are in, payers will realise their characters will step back from the edge and, if they tell you the cult is being evil making them make these decisions, yes, yes it is, thats why they where sent to stop it. 

Xalthu has pretty much summed up the options.

My call would be option 3 -- if they want to FAKE being evil to defeat the cult, I'm fine with that. If they want this to be a campaign about actually BEING evil and turning into the bad guys, then they can find another DM, because I am not down for that sort of RP.

But it's up to each DM how to handle this.