The whole batch … is holy: This is because the first part given as an offering is holy. This imagery comes from Numbers 15:17–21. In that passage, God told the Israelites to take part of their first batch of dough and set it apart as an offering to him.
Paul uses this image to explain God’s faithfulness to Israel. God’s promises to Abraham, Isaac, and Jacob were like a first portion given to God. Because of these promises, God’s work with his people will continue until it is complete.