Mark and avoid heretics

Romans 16:17 KJV  Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them.
Romans 16:18 KJV  For they that are such serve not our Lord Jesus Christ, but their own belly; and by good words and fair speeches deceive the hearts of the simple.

1 Corinthians 11:19 KJV  For there must be also heresies among you, that they which are approved may be made manifest among you.