1 John 4:3
And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
Cross-reference
1 John 4:2 gives the positive test — confessing Jesus incarnate — directly opposite the spirit that denies Him here.
In 1 John 2:18, the author says many antichrists have already come, grounding the statement that the antichrist spirit is already in the world.
In 1 John 2:22, the antichrist is defined as denying Jesus is the Christ — directly explaining the spirit that does not confess Jesus.
In 1 John 3:10, children of God are distinguished by righteousness — here by confession. Both test who belongs to God.
In 2 John 1:7, deceivers who deny Jesus came in the flesh are identified as antichrist — specifying the confession in 1 John 4:3.
In John 1:14, the Word became flesh — the very truth that the antichrist spirit denies, as referenced in 1 John 4:3.
In 1 Corinthians 12:3, speaking by the Spirit means confessing Jesus is Lord — directly supporting the test of spirits in 1 John 4:3.
In Romans 1:3, Jesus is descended from David according to the flesh — supporting the confession that Jesus came in the flesh.
Hebrews 10:5 quotes 'a body you prepared for me' — the incarnation the antichrist spirit denies. Connects confession to Christ's humanity.