You conveniently skipped the time line of events.
OK, the churches were destroyed first. So what?
Polish nation
What Polish nation? Polish peasants oppressed Ukrainian peasants? Because overwhelming majority of victims in Kresy were ordinary Poles, farmers and their families, slaughtered by their Ukrainian neighbours in the 20th century because of what? Wiśniowiecki and Chmielnicki?
oppressed the Ukrainian nation for centuries
You can't oppress something that doesn't exist. Ukrainians as a nation in modern sense are a 20th (OK, maybe 19th) century invention.
Ukrainians retaliated in the most horrible way
... and commited genocide that they deny. That's the difference, you see - Poles know about the destruction of orthodox churches and Piłsudski's betrayal of Petlura, nobody denies that. Ukrainians, on the other hand, refuse to acknowledge not only genocide but even the ethnic cleansing, and they do exactly what you do - justify horrible murderous atrocities with those vague "centuries of oppression".
Ireland was oppressed by England for 800 years - would it be OK for the Irish to mass murder English women and children in the middle of the 20th century for that? Finland was under Swedish occupation for 700 years, but somehow Finnish peasants didn't go on a rampage slaughtering their Swedish neighbours.
Kneel down together with Iron and beg for forgiveness.
What for? Ukrainians are not going to apologise for something they even refuse to acknowledge anyway. That's why your whole Let's-Apologise-First crusade is nonsensical.
In other words, you are the most vile apologist for ethnic cleansing.
Unfortunately, it looks like he is. Justifying Ukrainians commiting genocide on their neighbours with whom they lived and worked by something that Polish ruling authorities did.