Per https://www.planobiblechapel.org/tcon/notes/html/nt/1john/1john.htm,
"John did not say that every spirit that denies  Jesus, but every spirit that does not confess Jesus (v. 3). Often heretical teaching masks its deviations from the truth by simply failing to affirm important biblical truth. Rather than proclaiming, "Jesus is not  the Christ," they fail to affirm that He is the Christ."

Intentional omission of truth is heretical teaching.

Omission of truth can be heretical teaching.