# it is by faith

"it is by faith that we receive the promise" or "we receive the promise by faith"

# in order that the promise may rest on grace

"so that what he promised might be a free gift" 

# those who are under the law

This refers to the Jewish people, who were obligated to obey the law of Moses.

# those who share the faith of Abraham

"those who believe as Abraham did"

# father of us all

Here the word "us" refers to Paul and includes all Jewish and non-Jewish believers in Christ. Abraham is the physical ancestor of the Jewish people, but he is also the spiritual father of those who have faith.