Many Yidden keep very strict kashrus. Many of these Yidden embarrass baalei teshuva and converts by not accepting food gifts from them, rejecting offers for Shabbos and Yomim Tovim, and do not allow them to use their cooking appliances for fear their kashrus is not up to the same level as their own. This occurs even when the baalei teshuva and convert is tznius, proves to keep kosher, and follows all observable halacha. These baalei teshuva and converts feel rejected from the community in these interactions and it negatively affects their opinion of the frum community at large and ahavas yisroel.
Which mitzvah is more important: keeping stringent kashrus or accepting a convert and avoiding embarrassing a fellow Jew?
I sense your pain and frustration, and many people make this mistake of misbalancing their will to be strict regarding their kashrus standards, and they don’t realize that it is insulting to other people. R’ Eliyahu Lopian used to say that when person accepts upon himself to act stringently regarding a certain halacha, he has to be careful “because every chumra (stringent approach) can come out being a kula (lenient way of acting) in a different area”. Very often people make the mistake of being machmir regarding kashrus, when indeed it is being meikel in regard to bein adam l’chaveiro (regarding the way they treat other people). To enforce what you are saying, there is a story told about R’ Moshe Feinstein who used to visit a certain very small hotel, that was basically a one man show, which was run by an older woman who was a widow. Once a year R’ Moshe would visit her hotel, and she was very complimented by it. However, the kashrus that she used wasn’t the highest, it was definitely kosher, but there were hashgacha with a higher standard. There was once a person who was R’ Moshe eating there, and he asked him, aren’t you afraid that the food over there might not be correctly kosher and on the proper standard? To this R’ Moshe replied “the questions that you have regarding the hashgacha will surely not render the food trief, however insulting a widow is a safek d’orayso!
On the other hand, baal teshuva etc., should also realize that when it comes to kashrus, people can sometimes lose themselves and don’t realize that although there is a particular product or hashgacha that they don’t eat, but those who do eat it are not eating trief. Often these people are not really knowledgeable in the intricacies of kashrus, and they just know that we don’t eat hashgacha X. They don’t at all mean to hurt anyone, and they try hiding the fact that they are uncomfortable about eating certain hashgachos. Additionally, when you do give them food or invite them to your home, it would be a good idea to first find out what hashgachos they do eat and accommodate them. When you make them food, make it only with ingredients that they would eat, and let them know that you made it in a way that they can enjoy it without a problem. This way it will be better for you and for them.