“If cessationism is true, then why does God seem to continue to work through Pentecostal churches?”
For anyone unfamiliar with the term, “cessationism” is the teaching that the Holy Spirit of God no longer gives some of the spiritual gifts he gave in the early days of the church, the most comprehensive lists of which may be found in Romans 12 and 1 Corinthians 12. Cessationists believe the Lord intended certain gifts only to operate in the churches for a short time until the fall of Jerusalem in AD70, and until the completion of the canon of scripture.
Anyway, you’re asking the right person. I happen to be a cessationist, though I don’t usually use the term. My reasons for believing certain gifts are no longer being given may be found here, here and here.