Probably the core disagreement your friends would have with Catholics is with salvation (or justification and sanctification I guess); God's removing of the condemnation and penalty of sin and making a person righteous and able to be with God through Christ's work.
They probably believe that people are saved by grace through faith. They also probably believe Catholics believe that salvation is by works (which would be an oversimplification, but works are part of the process for Catholics).
Synergysm vs monergism.
The sacraments are necessary to salvation for Catholics.
Or at least what I was both taught at Catholic school and later what my evangelical teachers told me what Catholics believe and why it's fundamentally different.
If the method of salvation is different then there isn't much common ground.
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Uncertainty is an uncomfortable position.
But certainty is an absurd one.
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