Here is an account of Spurgeon's visit to a Catholic service in Belgium, when he visited there with his wife in 1860:
In Brussels, I heard a good sermon in a Romish church. The place was crowded with people, many of them standing,though they might have had a seat for a halfpenny or a farthing; and I stood, too; and the good priest — for I believe he is a good man, — preached the Lord Jesus with all his might. He spoke of the love of Christ, so that I, a very poor hand at the French language, could fully understand him, and my heart kept beating within me as he told of the beauties of Christ, and the preciousness of His blood, and of His power to save the chief of sinners.
He did not say, ‘justification by faith,’ but he did say, ‘efficacy of the blood,’ which comes to very much the same thing. He did not tell us we were saved by grace, and not by our works; but he did say that all the works of men were less than nothing when brought into competition with the blood of Christ, and that the blood of Jesus alone could save.
True, there were objectionable sentences, as naturally there must be in a discourse delivered under such circumstances; but I could have gone to the preacher, and have said to him, ‘Brother, you have spoken the truth;’ and if I had been handling the text, I must have treated it in the same way that he did, if I could have done it as well.
I was pleased to find my own opinion verified, in his case, that there are, even in the apostate church, some who cleave unto the Lord, — some sparks of Heavenly fire that flicker amidst the rubbish of old superstition, some lights that are not blown out, even by the strong wind of Popery, but still cast a feeble gleam across the waters sufficient to guide the soul to the rock Christ Jesus.” (Quoted in Lewis Drummond, Spurgeon: Prince of Preachers, 343-344).
This came from Mouw's Musings.