In the 1st letter of St John a person who does not recognize that Jesus is the Son of God is called the Antichrist. Are all Muslims Antichrists?

Answer: The expression Antichrist only appears in the letters of St John as a description of those who denied Christ in those days (2 John 7; 1 John 2:18b-22), or for Christ’s adversary who is expected at the end of time (1 John 2:18a; 4:3). In Catholic exegesis the latter is the same as the man of sin, the son of lawlessness, who opposes and exalts himself over everything that is called God, or is worshipped. He even sets himself up in God's temple, proclaiming himself to be God. He is the lawless one, whom the Lord (at his Second Coming) will overthrow with the breath of his mouth and destroy by the splendor of his coming. His coming will be in accordance with the work of Satan displayed in all kinds of counterfeit miracles, signs and wonders, and in every sort of evil that deceives those who are perishing. … (2 Thessalonians 2:3-10).

Nos 675-677 of the “Catechism of the Catholic Church” (CCC) speak of the final trial of the Church and, in this context, also of the Antichrist.

“675 Before Christ's second coming the Church must pass through a final trial that will shake the faith of many believers [cf. Luke 21:12); John 15:19-20]. The persecution that accompanies her pilgrimage on earth will unveil the "mystery of iniquity" in the form of a religious deception offering men an apparent solution to their problems at the price of apostasy from the truth. The supreme religious deception is that of the Antichrist, a pseudo-messianism by which man glorifies himself in place of God and of his Messiah come in the flesh (cf. 2 Thessalonians 2:4-12); 1 Thessalonians 5:2-3); 2 John 7; 1 John 2:18-22].

676 The Antichrist's deception already begins to take shape in the world every time the claim is made to realize within history that messianic hope which can only be realized beyond history through the eschatological judgment. The Church has rejected even modified forms of this falsification of the kingdom to come under the name of millenarianism [cf. DS3839], especially the "intrinsically perverse" political form of a secular messianism. [cf. [cf. the rejection of false mysticism, the erroneous form of salvation of the poor in the Encyclical Divini Redemptoris" Pius XI.; cf. Also Gaudium et Spes, 20-2].

677 The Church will enter the glory of the kingdom only through this final Passover, when she will follow her Lord in his death and Resurrection [cf. GS 20-21]. The kingdom will be fulfilled, then, not by a historic triumph of the Church through a progressive ascendancy [cf. Revelation 19: 1-9], but only by God's victory over the final unleashing of evil [cf. Revelation 2:18], which will cause his Bride to come down from heaven [cf. Revelation 20:7-10]. God's triumph over the revolt of evil will take the form of the Last Judgment after the final cosmic upheaval of this passing world [cf. 2 Peter 3:12-13].”