A little leaven spoils the whole loaf so they should be ignored altogether even if they have a few good things to say.
If we fellowship and accept those that teach false doctrine then other weaker brothers see that thinking it's OK to fellowship with them too and then they become deceived and end up in hell with the false teachers - now we are responsible for allowing weaker brothers to be deceived having played a part on their deception.
That's why we need to call out the false doctrines as being false to warn all, including weaker brothers so in the end if they get deceived it's not on us as we warned them.
Heresy (false doctrine) is one of the
works of the flesh that cause some to not be accepted in God's Kingdom so this business of thinking the calvinists are still saved is a trick of the devil in his attempt to get God's Children to accept the tares and start believing their false doctrine so they too go to hell along with the tares.
Let him that has ears to hear take heed and depart from the false teachers and warn others of their false teachings to help others not fall in to satan's trap of nice sounding, sincere sounding doctrines that is actually doctrines of demons.