Tafsir of Al-Kahf 18:20

إنهم إن يظهروا عليكم يرجموكم

and let no man know of you. For, if they come to know of you, they will stone you, means, `if they find out where you are,'

يرجموكم أو يعيدوكم في ملتهم

they will stone you or turn you back to their religion; They were referring to the followers of Decianus, who they were afraid might find out where they were, and punish them with all kinds of torture until they made them go back to their former religion, or until they died, for if they agreed to go back to their (old) religion, they would never attain success in this world or the Hereafter. So they said:

ولن تفلحوا إذا أبدا

and in that case you will never be successful