"whose God ?"
There is only one God, the creator of the heavens and the earth:
Ps 95:5 For all the gods of the Gentiles* are devils: but the Lord made the heavens.
St. Robert Bellarmine, S.J., commentates on this verse:
St. James [2:19] says, “the devils also believe and tremble;” and, as David alludes to false gods, especially in this Psalm, he, therefore, assigns a reason for our God being feared above all gods, when he says, “For all the gods of the gentiles are devils; but the Lord made the heavens;” that is to say, God is to be feared above all false gods, erroneously adored by the gentiles, because the gods of the gentiles are not true gods, but demons, who, through pride, have revolted from the God who created them, and have been doomed by him to eternal punishment; “but the Lord,” instead of being a spirit created, is a creating spirit, who “made the heavens,” the greatest and the most beautiful things in nature, as well as everything under its canopy, that is, all things created.
Worshiping false gods is the sin of idolatry.
False gods are airy nothings:
Pohle's Dogmatic Treatise vol. 1:
The Book of Wisdom devotes several chapters76 to the refutation and condemnation of Polytheism and Idolatry. In fact, Holy Scripture never tires of denouncing Idolatry as foolish and impious, and the pagan deities as “not gods,”77 “lies and vanity,”78 “wind and vanity,”79 airy nothings.80
76. Wisd. 12-15.
77. 2 Kings 19:18; Jer. 2:11.
78. Jer. 16:19.
79. Is. 41:24; Dan. 5:23.
80. Ps. 95:5; not אֶלוֹהים, but אֱלִילִים, i. e., nihila ["nothings"].
To worship them violates the First Commandment:
Exodus 20:2 I am the Lord thy God, who brought thee out of the land of Egypt, out of the house of bondage.
3 Thou shalt not have strange gods before me.
In the ¶ after what I quoted in my answer to "Do Catholics view atheism as a sin?", St. Thomas Aquinas writes (Summa Theologica II-II q. 10 a. 3 co.):
Nor is it possible for one who has a false opinion of God, to know Him in any way at all, because the object of his opinion is not God.