Welcome. What you’ve been taught is quite contrary to what the Scriptures teach. You’ve isolated one text and ignored the rest.
Purgatory does not deny the Salvation of the Cross by any means. If you read through scripture, you will see that it teaches us much more than John 3:16, Jesus commands Baptism, the Apostles teach Jesus’ instructions when they tell us that we should work out our salvation with fear and trembling. We also have our Lord’s teachings telling us that our souls will be tried as by a refiner’s fire. Rather than the Protestant error that not only is the guilt of sin forgiven by Christ, but the restitution to our neighbor and God for the offence is blotted out, the Catholic Church hears Jesus when in
When the founders of various Protestant Sects decided that they and only they understood the Truth, and that all before them did not, they selected proof texts and rejected others that did not fit their presonal teachings, If you look at what the earliest Christians believed and died for, you will find it lines up with what the Catholic Church has taught since founded on the Apostles by Jesus Himself. Rather than placing your faith in one verse in the Scriptures, I’d ask you to place it in Jesus’ teachings, the entirety still preached by the Catholic Church, through which He works even today. As promised by our Lord, the Church which He would remain with even to the end of time.