Donatism is the erroneous belief that the sacraments' effectiveness depends on the minister's moral character. It teaches that baptism would be invalid if a minister involved in sin were to baptize a person.
The problem with this error is given to us in Romans.
"for all have sinned and fall short of the glory of God" Romans 3:23
No one is morally pure, for we have all sinned. Therefore, the effectiveness of the baptism or the administration of the Lord's supper does not cease to be effective if the moral character of the minister is in question or even demonstrated to be faulty. Instead, the sacraments are powerful because of what they are; visible or outward representations of spiritual realities.
God alone is the one who works in and through them, and The moral state of the administrator does not restrict him.