1 John 4:2

Hereby--"Herein." know . . . the Spirit of God--whether he be, or not, in those teachers professing to be moved by Him. Every spirit--that is, Every teacher claiming inspiration by the HOLY SPIRIT. confesseth--The truth is taken for granted as established. Man is required to confess it, that is, in his teaching to profess it openly. Jesus Christ is come in the flesh--a twofold truth confessed, that Jesus is the Christ, and that He is come (the Greek perfect tense implies not a mere past historical fact, as the aorist would, but also the present continuance of the fact and its blessed effects) in the flesh ("clothed with flesh": not with a mere seeming humanity, as the DocetÃ&brvbr afterwards taught: He therefore was, previously, something far above flesh). His flesh implies His death for us, for only by assuming flesh could He die (for as God He could not), Heb 2:9-10, Heb 2:14, Heb 2:16; and His death implies His LOVE for us (Joh 15:13). To deny the reality of His flesh is to deny His love, and so cast away the root which produces all true love on the believer's part (Jo1 4:9-11, Jo1 4:19). Rome, by the doctrine of the immaculate conception of the Virgin Mary, denies Christ's proper humanity.

— Jamieson-Fausset-Brown Commentary (public domain)