“Fr. D. Prummer OP, writing in 1910, affirms in his Manuale Theologiæ Moralis that it is never licit for a Catholic to take part in a non-Catholic cult with the intention of worshipping God in the manner of non-Catholics, more acatholicorum. Such an act, he declares, is nothing other than a denial of the Catholic faith.2 In the same year, writing an article on ‘Heresy’ for the Catholic Encyclopœdia, Fr. J. Wilhelm SJ affirms that a Catholic may attend non-Catholic services, but only ‘provided no active part be taken in them’. In an article on the same subject, the Dictionnaire de théologie catholique reiterates, in 1920, that active participation in non-Catholic rites is toujours interdite – the reason being that it is ‘equivalent to a denial of the Catholic faith’. In 1930, Fr. B. Merkelbach OP in his Summa Theologiæ Moralis writes that ‘active participation in the sacred things of a [non-Catholic] public cult is illicit, since it implies approval of the worship and a recognition of the sect.’3 Using a slightly different terminology but teaching the same doctrine, Fr. L. Fanfani OP writes, in 1950, ‘material communicatio in sacris [‘material’ in the sense that the person in question does not mean to renounce his Catholic faith], if it is active and immediate, is never permissible for Catholics.’4 The reason for this, he explains, is that such behaviour necessarily manifests a commitment to a heretical or at least an illegitimate cultus.”
http://www.christendom-awake.org/pages/thomas-crean/praying-with-non-catholics.htm