Therefore I inform you that no one who is speaking by the Spirit of God says, “Jesus be cursed,” and no one can say, “Jesus is Lord,” except by the Holy Spirit.
So Paul tells them that no one speaking by the Spirit of God says, "Jesus be cursed," and no one can say, "Jesus is Lord," except by the Holy Spirit.
It gives the foundational Christological marker of the Spirit's work.
1Now about spiritual gifts, brothers, I do not want you to be uninformed. 2You know that when you were pagans, you were influenced and led astray to mute idols. 3Therefore I inform you that no one who is speaking by the Spirit of God says, “Jesus be cursed,” and no one can say, “Jesus is Lord,” except by the Holy Spirit. 4There are different gifts, but the same Spirit. 5There are different ministries, but the same Lord. 6There are different ways of working, but the same God works all things in all people. 7Now to each one the manifestation of the Spirit is given for the common good. 8To one there is given through the Spirit the message of wisdom, to another the message of knowledge by the same Spirit, 9to another faith by the same Spirit, to another gifts of healing by that one Spirit, 10to another the working of miracles, to another prophecy, to another distinguishing between spirits, to another speaking in various tongues, and to still another the interpretation of tongues. 11All these are the work of one and the same Spirit, who apportions them to each one as He determines.