Indeed, if they come to know of you, they will stone you (18:20)

﴿إِنَّهُمْ إِن يَظْهَرُوا عَلَيْكُمْ يَرْجُمُوكُمْ أَوْ يُعِيدُوكُمْ فِي مِلَّتِهِمْ وَلَن تُفْلِحُوا إِذًا أَبَدًا﴾
[ الكهف: 20]

18:20 Indeed, if they come to know of you, they will stone you or return you to their religion. And never would you succeed, then - ever."

If your people discover you and find out where your whereabouts, they will kill you by stoning you, or they will force you to return to their deviated creed that you used to follow before Allah favoured you by guiding you to the true religion.
If you return to that you will never succeed - not in this world nor in the Hearafter.
Instead, you will suffer a great loss in both places on account of your leaving the true religion to which Allah had guided you and your returning to that deviated creed.

"For if they come to know of you, they will stone you (to death or abuse and harm you) or turn you back to their religion, and in that case you will never be successful."

phonetic Transliteration

Innahum in yathharoo AAalaykum yarjumookum aw yuAAeedookum fee millatihim walan tuflihoo ithan abadan

Abdullah Yusuf Ali - Translation

"For if they should come upon you, they would stone you or force you to return to their cult, and in that case ye would never attain prosperity."

Safi-ur-Rahman al-Mubarakpuri

"For, if they come to know of you, they will stone you or turn you back to their religion; and in that case you will never be successful."

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