Jamieson, Fausset and Brown on 1 Corinthians 12:3

3. The negative and positive criteria of inspiration by the Spirit--the rejection or confession of Jesus as Lord [Alford] (1Jo 4:2; 5:1). Paul gives a test of truth against the Gentiles; John, against the false prophets.

by the Spirit--rather, as Greek, "IN the Spirit"; that being the power pervading him, and the element in which he speaks [Alford], (Mt 16:17; Joh 15:26).

of God ... Holy--The same Spirit is called at one time "the Spirit of God"; at another, "the HOLY Ghost," or "Holy Spirit." Infinite Holiness is almost synonymous with Godhead.

speaking ... say--"Speak" implies the act of utterance; "say" refers to that which is uttered. Here, "say" means a spiritual and believing confession of Him.

Jesus--not an abstract doctrine, but the historical, living God-man (Ro 10:9).

accursed--as the Jews and Gentiles treated Him (Ga 3:13). Compare "to curse Christ" in the heathen Pliny's letter [Epistles, 10.97]. The spiritual man feels Him to be the Source of all blessings (Ep 1:3) and to be severed from Him is to be accursed (Ro 9:3).

Lord--acknowledging himself as His servant (Is 26:13). "Lord" is the Septuagint translation for the incommunicable Hebrew name Jehovah.