“There is no other head of the Church but the Lord Jesus Christ: nor can the Pope of Rome in any sense be head thereof; but is that Antichrist, that man of sin, and son of perdition, that exalteth himself in the church against Christ, and all that is called God” (WCF 25.6).
The prooftexts for this claim are Mt 23:8-10; 2 Thes 2:1-12, and Rev 13. I’ve argued elsewhere that these passages of Scripture cannot be treated as directly prophetic of the papacy because the language is too general and flexible to single out the institution of the papacy in particular.
That doesn’t mean that they are inapplicable to the papacy. Rather, they are applicable to any individual or institution which exemplifies these characteristics.
But one can approach this question from another angle as well. In modern parlance, “anti” means “opposed to,” but in Greek, “anti” means “in place of.”
Now, the NT speaks of the exclusive mediation of Christ. The locus classicus is 1 Tim 2:5-6. The same idea is present in Johannine theology (e.g. Jn 14:6; 1 Jn 2:1-2; 4:14). And the entire argument of Hebrews is devoted to this theme.
But the Pope claims to be the Vicar of Christ. As such, he acts in place of Christ. Indeed, the entire system of Romanism, with its cult of the saints, culminating in Mary as the Co-Redemptrix and Mediatrix, as well as a priesthood which dispenses the means of grace, embodies the principle of the Antichrist.
This is not to limit the principle to Catholicism. You have the functional equivalent in Buddhism and Islam and Mormonism, among others.