I believe that public school was sold to protestants as an anti-Catholic thing but it was really intended to be an anti-Christian thing.
Many forces shaped compulsory government schooling, not the least of which was anti-Catholicism. The anti-Catholic Know Nothing movement was very powerful at the time of the establishment of compulsory schooling laws in Massachusetts. The laws followed on the heels of mass Irish Catholic immigration to Boston in the 1840s and 1860s.
The purpose of the compulsory attendance laws was to force the children of Irish Catholic immigrants into the Protestant government schools. In reaction to the compulsory attendance laws, Catholic bishops established Catholic schools for Catholic children.
Do you remember this from your government school history textbooks? I don't.
During the half-century preceding the Knownothing era, the questions involved in that movement had been frequently agitated. Catholics and foreigners were denounced, mainly from Protestant pulpits, as enemies of the Republic. Books and newspapers calculated to inflame the passions of the mob against their Irish and Catholic neighbours were extensively circulated. Catholic bishops and priests were maligned, their religion misrepresented and ridiculed, and acts of violence were committed against Catholics and their property. The burning of the Convent of the Ursuline nuns at Charleston, Mass., in 1834, by a Native American mod, and their cruel treatment of the unoffending nuns and their pupils, were the most notable manifestations, up to that time, of the evil effect of religious hatred. In 1835 the first formal organization of the partisans of the Native American movement under that name, was effected at New York City. Various newspapers, such as "The Protestant", "The Protestant Vindicator", "The Downfall of Babylon", and the like, were established in New York and New England as aids to the movement. The "evils of Popery" and the danger to arise to the Republic from tolerating the practice of the Catholic religion were staple topics of discussion by no inconsiderable number of ministers of religion, and under the impulse of these incitements the spirit of religious prejudice was kept alive; there were new aggressions upon the rights of Catholic citizens, the peace and order of the community were occasionally disturbed by acts of violence, and another decade in the history of Native Americanism terminated in the bloody riots which occurred at Philadelphia, in 1844, when several Catholic churches were attacked by the Native American mob, and two of them, St. Michael's and St. Augustine's, were deliberately fired and reduced to ashes, and the safety of those that remained were so endangered by the hostile demonstrations of the mob that public worship was suspended by order of Bishop Kenrick, and on Sunday, 12 May, 1844, all Catholic churches in that city were closed. Many houses tenanted by Irish Catholics were likewise wantonly destroyed by fire, some of the inmates were shot down at their doorsteps, and a number of other unoffending citizens lost their lives.The party whose members were soon to be described as "Knownothings" was formally organized in 1852 in the City of New York. Although begun as a local society, it was designed to become a national organization. Its leaders had planned to concentrate in a single party the membership of various Native American orders already in existence and the "American Republicans", the "Order of United Americans", "Sons of America", and "United American Mechanics of the United States" formed the nucleus of the new party. It adopted the title of "National Council of the United States of North America." Among the initiate it was called the "Supreme Order of the Star-spangled Banner" and was sometimes familiarly spoken of as "Sam". Its published ritual declared (Article II) the purpose of the organization to be "to protect every American citizen in the legal and proper exercise of all his civil and religious rights and privileges; to resist the insidious policy of the Church of Rome and all other foreign influence against our republican institutions in all lawful ways; to place in all offices of honour, trust or profit in the gift of the people or by appointment none but Native American Protestant citizens" (see American Politics, Book I, pp. 57-9). Article III declared "that a member must be a native-born citizen, a Protestant either born of Protestant parents or reared under Protestant influence, and not united in marriage with a Roman Catholic. . .no member who has a Roman Catholic wife shall be eligible to office in this order", etc.
...In 1854 the Knownothings sent forty representatives to Congress, and elected their candidate, Gardiner, Governor of Massachusetts, with a legislature of the same type. In New York, in the same year, the party polled 122,000 votes and in the next year was successful in that state, polling 146,000 votes. In 1855 New Hampshire, Connecticut, and Rhode Island each elected a Knownothing governor, and the party carried the elections in nine different states.
...In 1853, on the occasion of the visit to America of Archbishop Bedini, Apostolic Nuncio to the Court of Brazil, a great outcry was raised by the Knownothing element throughout the country, with whom were joined certain Italian refugees who had emigrated to escape the consequences of their criminal conduct at home. In all the cities visited by the archbishop hostile demonstrations were made against him. At Boston, Baltimore, Wheeling, St. Louis, and Cincinnati where the Nuncio took part in various solemn religious celebrations, there were scenes of disorder, and in some cases of bloodshed, provoked by the Knownothing speakers both lay and clerical, as well as by the anti-Catholic press. At Cincinnati, in December, 1853, a mob of 600 men armed with weapons of various sorts, and carrying lighted torches and ropes, marched to the cathedral intending to set it on fire and, as was believed, to hang the Nuncio. There was an encounter with the police, and the mob was dispersed, but not until after shots had been fired and several persons wounded. During 1854 there were numerous assaults upon Catholic churches throughout the country by the Knownothing element. St. Mary's church at Newark, N. J., was invaded by a mob made up of Knownothings and Orangemen from New York City; the windows were broken, some of the statuary destroyed, and one unoffending bystander, an Irish Catholic, was shot and killed. In October of the same year, at Ellsworth, Maine, Father John Bapst, S.J., was dragged from the church, robbed of his watch and money, tarred and feathered, and ridden about the village on a rail. On 4 July, at Manchester, N. H., St. Anne's church was attacked, its windows broken and furniture destroyed, the priest compelled to seek shelter away from his home, and the houses of Irish Catholics were likewise attacked, the inmates driven out, even the sick being dragged from their beds. At Bath, Me., the mob broke into the church and, after wrecking the altar and the pulpit, set fire to the building which was reduced to a heap of ashes. At Dorchester, Mass., a keg of gunpowder was placed under the floor of the little Catholic church, it was fired at three o'clock in the morning and resulted in almost the total destruction of the building. Another Catholic church, at Sidney, Ohio, was blown up with gunpowder. At Massillon, Ohio, another church was burned, and an attempt made to burn the Ursuline Convent at Galveston, Texas. At Lawrence and at Chelsea, Mass., the Catholic churches were attacked by the Knownothing mob, the windows smashed, and much other damage done. St. Mary's church at Norwalk, Conn., was set on fire and later its cross was sawed off the spire. A fire was started in the church of Sts. Peter and Paul in Brooklyn, and the building was saved only by the interference of the police aided by the militia, who drove off the mob. St. Mary's Church at Saugerties, N. Y., was set on fire and nearly destroyed by the fanatics, and an attempt made to burn the church at Palmyra, N. Y. The following year (1855), at Louisville, Ky., the elections were attended with such rioting and bloodshed, the result of Knownothing agitation, that the day (5 Aug.) acquired the name of "Bloody Monday". The cathedral was invaded by the mob and was saved from destruction only by the prudence of Bishop Spalding, who, in a letter to Bishop Kenrick summing up the results of the day's proceedings, said: "We have just passed through a reign of terror surpassed only by the Philadelphia riots. Nearly one hundred poor Irish have been butchered or burned and some twenty houses have been consumed in the flames. The City authorities, all Knownothings, looked calmly on and they are now endeavouring to lay the blame on the Catholics" (see "Life of Archbishop Spalding", by J.L. Spalding, p. 185).