Worshipping with Protestants Is A Mortal Sin

No one can find salvation except in the Catholic Church. Outside the Church, you can find everything except salvation. You can have dignities, you can have Sacraments, you can sing "Alleluia," answer "Amen," have the Gospels, have faith in the name of the Father, the Son, and the Holy Ghost, and preach it, too. But never can you find salvation except in the Catholic Church - St. Augustine

There Is Only One True Religion

The Church established by our Lord Jesus Christ has always taught that organized religion is revealed by God and necessary for salvation. And the one religion revealed by God that is true, which is confirmed by both reason and countless verifiable miracles, is the Catholic Religion. There is no salvation found in any other religion. Just as there is only one God who is the only way to salvation (cf. John 14:6) there is only one religion established by Him for our salvation. Any other religion must necessarily be a false religion since its teachings contradict the Catholic religion. 

Logically speaking, there can, at most, be only one true religion. And the disunity and contradictions among the thousands of Protestant denominations violate the very principle of catholicity - not to mention apostolicity - which must be found in the true religion. With the advent of Protesantism in the 1500s, Luther stated that the Bible was open to individual interpretation, and the theological trail became crisscrossed with Biblical theorizing and harsh denunciations. Luther, Calvin, Zwingli, Anabaptists, and others all preached different pathways of what each described as the true road to salvation. None of them agreed. And none of their respective denominations agree. They all conflicted and continue to conflict with each other. Just as 1 + 1 must equal 2, it is impossible for all of the conflicting and varied protestant groups to all be true. The truth is actually found only in the Catholic Church.

Worshipping With Other Religions Is a Mortal Sin Against the First Commandment

The First Commandment enjoins us not only to believe in God and worship Him but also to trust Him and love Him, as well as to encourage others to do likewise. The Catechism of the Council of Trent explained this well when discussing the First of the 10 Commandments:

The (mandatory part) contains a precept of faith, hope and charity. For, acknowledging God to be immovable, immutable, always the same, we rightly confess that He is faithful and entirely just. Hence in assenting to His oracles, we necessarily yield to Him all belief and obedience. Again, who can contemplate His omnipotence, His clemency, His willing beneficence, and not repose in Him all his hopes? Finally, who can behold the riches of His goodness and love, which He lavishes on us, and not love Him? Hence the exordium and the conclusion used by God in Scripture when giving His commands: I, the Lord.

Next, the Catechism explains what the First Commandment forbids:

The (negative) part of this Commandment is comprised in these words: Thou shalt not have strange gods before Me. This the Lawgiver subjoins, not because it is not sufficiently expressed in the affirmative part of the precept, which means: Thou shalt worship Me, the only God, for if He is God, He is the only God; but on account of the blindness of many who of old professed to worship the true God and yet adored a multitude of gods. Of these there were many even among the Hebrews, whom Elias reproached with having halted between two sides [III Kngs xviii. 21], and also among the Samaritans, who worshipped the God of Israel and the gods of the nations [cf. IV Kngs xvii. 33].

Consequently, sins against the First Commandment include (among other things) failing to pray, failing to study the Faith, neglecting spiritual duties, taking part in the worship of non-Catholic religions, despair, presumption, idolatry, consulting fortune tellers, and observing superstitious practices like horoscopes. The Baltimore Catechism clearly reiterates this prohibition against spells and charms: “Those who make use of spells and charms, or who believe in dreams, in mediums, spiritists, fortune-tellers, and the like, sin against the First Commandment, because they attribute to creatures perfections which belong to God alone.” 

Worshiping with Protestants Is A Mortal Sin

Whispers in Restoration summarized the rationale for why active participation in non-Catholic (e.g., Protestant worship) is always forbidden. Similarly, assisting at Jewish seder meals and any other religious ceremony of non-Catholics is also strictly forbidden:

In considering the question of Catholics joining in non-Catholic worship, the constant and uniform testimony of Scripture and Tradition must be maintained: Catholics may never actively participate in non-Catholic worship. This prohibition follows chiefly from the First Commandment in light of the fact that all non-Catholic worship is false, actions standing contrary to right faith and in violation of both natural and divine law. Such acts are therefore objectively disordered, independent of the subjective culpability of those who engage in such worship.

A second, closely connected reason for this discipline is that of making a lie by demonstrating a false religious unity: for a Catholic to join in non-Catholic worship is to manifest a certain unity with that community, contradicting the true unity of the Church. This leads to a third reason of scandal: Catholics who actively engage in false worship give the objective impression that such disordered acts are permissible, even laudable, and in this way endanger right faith (on the part of Catholics) and confirm non-Catholics in their error. A final reason for this prohibition is that it involves an omission of fraternal charity: by engaging in false worship, the Catholic fails in his duty to mercifully instruct the ignorant, admonish the sinner, and share the Gospel.

It is therefore doctrinally indefensible to admit of a discipline – alien to the constant and uniform tradition of the Church – that would permit (much less encourage) the active participation of Catholics in non-Catholic worship. 

This prohibition was also clearly specified in Canon Law: “It is unlawful for the faithful to assist in any active manner, or to take part in the sacred services of non-Catholics” (1917 Code of Canon Law, Canon 1258, Paragraph 1). But this teaching long pre-dated the 1917 Code and was declared at multiple Councils:

“If any ecclesiastic or layman shall go into the synagogue of the Jews or the meeting-houses of the heretics to join in prayer with them, let them be deposed and deprived of communion. If any bishop or priest or deacon shall join in prayer with heretics, let him be suspended from communion.” (3rd Council of Constantinople, 680 AD).

“And since truth cannot contradict truth, we define that every statement contrary to the enlightened truth of the faith is totally false and we strictly forbid teaching otherwise to be permitted. We decree that all those who cling to erroneous statements of this kind, thus sowing heresies which are wholly condemned, should be avoided in every way and punished as detestable and odious heretics and infidels who are undermining the Catholic faith.” (Session 8 of the 5th Lateran Council, 1513 AD)

Several valid but non-Ecumenical councils further affirmed this same truth (along with other examples):

“No one shall pray in common with heretics and schismatics… It is not permitted to heretics to enter the house of God while they continue in heresy.” (Council of Laodicea during the 4th century, citing Canon 6)

“One must neither pray nor sing psalms with heretics, and whosoever shall communicate with those who are cut off from the communion of the Church, whether clergy or layman, let him be excommunicated.” (Council of Carthage)

Can Catholics Attend Non-Catholic Funeral or Wedding Services?

What about non-passive worship? The author from Whispers in Restoration continues by noting that even passive (i.e., non-active) attendance at non-Catholic worship (e.g., at a funeral or wedding of a non-Catholic) would be limited and one which should be considered with the prudential advice of a Catholic priest:

Furthermore, it should be noted that traditionally, if Catholics might be permitted a certain passive participation in occasions of false worship, this was admitted only if the instance was: 1) an extraordinary circumstance, 2) commended by some grave reason, and 3) not overtly scandalous. The cautious qualifications here reflect the gravity of the act in question and recognition of the fact that any form of worship is informed by the beliefs of the worshipping community, demonstrating and effecting their religious unity as well. Thus for a Catholic, even passive participation in non-Catholic worship is a question that must be weighed with great caution.

One typical example given for such potentially permissible passive participation is that of a Catholic attending the non-Catholic funeral of a close relative or friend, provided that fraternal charity truly compels it and there be no danger of scandal or harm to right faith. Even here, it is noteworthy that such participation was only ever admitted as a possibility, and on the assumption that the person was seeking the direction of legitimate pastors in good faith, in order to act well.

Conclusion

We must reject religious indifferentism and seek to win as many souls from the devil and error as possible. By keeping the First Commandment and encouraging others to become Catholic, we observe this first and foundational Commandment. As such, no Catholic may ever participate in the religious worship of any non-Catholic religion—even those of Protestants. Catholics may only actively participate in Catholic worship without exception. Even if a family member goes to such a non-Catholic place of worship, the Catholic is not permitted to join, and if he does, he commits a serious mortal sin and risks excommunication.