This is my understanding and it is also thought in the early Church fathers (no, they were not Catholic at least until ca 4th century) like Tertulian (later fathers I reject and I do believe this to be a Baptist thinking).
When we see references to Noe, Lot and Rehab we see that the LORD has always delivered His people and then brought the judgement upon the ungodly. Also Apostle Paul told us that we have passed from the wrath of God and also Jesus is telling the Philadelphian Church that because she has keept the Word she shal be saved from the hour of temptation that will come upon the whole world. So this are clear teaching of the pre-19th century pre-trib Rapture.
Also I've seen an intereting video where the presentator quotes from the Catholic cathecism which says that the church (in their opinion) has to go trought the tribulation so her faith will be tested. Why would her faith be tested if we are saved by God's grace trough faith and not by our own efforts?! Also the presentator says it nicely: think about it if there is post trib Rapture what will you be doing: building a bunker in the mountains or preach the gospel? Now ask yourself this same for the pre-trib: there is no need for a bunker so your time can be spent on preaching.
About saints going trough tribulation already: true, millions have suffered and are suffering but the Bble speakins about THE Tribulation, not local tribulations. Think about that and may the LORD give us all one perfect understanding.