You shall not make a graven image for yourself, or any likeness in the heavens above, or in the earth beneath, or in the waters under the earth; Exodus 20:4 KJ3. (Levitus 26:1).
Following the religions of the other nation is called idolatry.
Hence, following Christianity which broke away from Judaism and distinguished herself with her own Christian sabbath in contrast to the sign of the seventh day sabbath (Exodus 31:16-17), her modern idolatry disguised in christened idols (images of so-called saints) and her own spiritual leaders instead of God Jesus Christ the Lord (John 8:24) is also fornication.
Ezekiel 16:26-29 ASV
(26) Thou hast also committed fornication with the Egyptians, thy neighbors, great of flesh; and hast multiplied thy whoredom, to provoke me to anger.
(27) Behold therefore, I have stretched out my hand over thee, and have diminished thine ordinary food, and delivered thee unto the will of them that hate thee, the daughters of the Philistines, that are ashamed of thy lewd way.
(28) Thou hast played the harlot [fornicated] also with the Assyrians, because thou wast insatiable; yea, thou hast played the harlot with them, and yet thou wast not satisfied.
(29) Thou hast moreover multiplied thy whoredom [porneia] unto the land of traffic, unto Chaldea; and yet thou wast not satisfied herewith.
Fornicating AFTER other gods and nations
Exodus 34:10-16 ASV
(10) And he said, Behold, I make a covenant: before all thy people I will do marvels, such as have not been wrought in all the earth, nor in any nation; and all the people among which thou art shall see the work of Jehovah; for it is a terrible thing that I do with thee.
(11) Observe thou that which I command thee this day: behold, I drive out before thee the Amorite, and the Canaanite, and the Hittite, and the Perizzite, and the Hivite, and the Jebusite.
(12) Take heed to thyself, lest thou make a covenant with the inhabitants of the land whither thou goest, lest it be for a snare in the midst of thee:
(13) but ye shall break down their altars, and dash in pieces their pillars, and ye shall cut down their Asherim;
(14) (for thou shalt worship no other god: for Jehovah, whose name is Jealous, is a jealous God);
(15) lest thou make a covenant with the inhabitants of the land, and they play the harlot [FORNICATE] after their gods, and sacrifice unto their gods, and one call thee and thou eat of his sacrifice;
(16) and thou take of their daughters unto thy sons, and their daughters play the harlot [FORNICATE] after their gods, and make thy sons play the harlot [FORNICATE] after their gods.