Bechorot 30 - People Who Are Untrustworthy
One who is suspect to flaunt the laws of firstborn is not trusted to buy even deer meat from him. It is likewise forbidden to buy raw hides from him, but tanned hides are permitted: since, if he were discovered, he would loose the hide, he won't put that much labor into the hide of a firstborn.
One who is suspect of violating the laws of the seventh year (shemittah) is nevertheless trusted regarding tithes. He may consider tithes more stringent, because they are eaten within the walls of Jerusalem. Conversely, one who is suspect regarding tithes is still trusted regarding shemittah: he may regard shemittah as more stringent, since the produce of shemittah cannot not be redeemed for money. Rabbi Yochanan remarked, "That is the opinion of Rabbi Akiva, who authored many anonymous rulings, but Rabbi Meir would say that one who is suspect regarding a single commandment is suspect regarding all."
If a convert accepted on himself the code of the Torah and then became suspect regarding only one aspect of it, he is considered suspect regarding all areas; however, his conversion is not invalidated. A Kohen who denies one commandment cannot serve in the Temple.
Art: Charles Olivier De Penne - The Deer Hunt