Devarim (Deuteronomy) 13:12 (11)
Vechol-Yisra’el yishme’u veyira’un velo-yosifu la’asot kadavar hara hazeh bekirbecha.
All Yisra’el shall hear, and fear, and shall not do any more such wickedness as this is in the midst of you.

The context of this pasuk (verse) is this:

Devarim (Deuteronomy) 13:7 (6)
Ki yesitcha achicha ven-imecha o-vincha o-vitcha o eshet cheykecha o re'acha asher kenafshecha baseter lemor nelchah vena'avdah elohim acherim asher lo yadata atah va'avoteycha.
If your brother, the son of your mother, or your son, or your daughter, or the wife of your bosom, or your friend, who is as your own soul, entice you secretly, saying, Let us go and serve other elohim, which you have not known, you, nor your fathers;

Therefore, the wickedness to be avoided here is enticing others to go after other elohim.

The word for entice in Hebrew is sit. It means to allure, convince, to stir up. The person who entices others to follow other elohim is called a meysit. The Targum Yonathan interprets this word as "to mislead."

There are Orthodox Jews that would apply this mitzvah to the missionary. (see Radak). They think that those telling them about Yeshua violates this mitzvah, when in truth, they are preaching about the Messiah. I would rather apply this mitzvah in a more literal meaning, simply that one should not entice others to worship other elohim, just as the text says.

2 Kefa (Peter) 2:1-3
But among the people there were also false prophets, just as there will be false teachers among you. Under false pretenses they will introduce destructive heresies, even denying the Master who bought them, and thus bring on themselves swift destruction. Many will follow their debaucheries; and because of them, the true Way will be maligned. In their greed they will exploit you with fabricated stories. Their punishment, decreed long ago, is not idle; their destruction is not asleep!

1 John 2:26-27
I have written you these things about the people who are trying to deceive you. As for you, the Messianic anointing you received from the Father remains in you, so that you have no need for anyone to teach you. On the contrary, as his Messianic anointing continues to teach you about all things, and is true, not a counterfeit, so, just as he taught you, remain united with him.

The Ruach HaKodesh (Holy Spirit) will help us know what the truth is.

Yochanan (John) 16:13
When the Ruach Emet (Spirit of truth) comes, he will guide you into all the truth; for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come.