God spoke to Abraham and Abraham heard and had faith in what God promised. God did not speak to Abraham at all about Jesus dying on the cross for his sins or about Jesus rising from the dead for his justification. Resurrection was certainly a known topic, but Abraham knew nothing of Jesus Christ and what His death, burial, and resurrection would accomplish. The fact of the matter is that the Bible does not record God having any discussion with Abraham about the man-God Jesus Christ and His death, burial, and resurrection.
Paul is making the point here in Galatians 3 that God new all of this information from the beginning and God spoke to Abraham, gave him a promise, and He knew that He would justify the heathen through faith, which what Paul preaches, but “in the shall all the families of the earth be blessed” is not “believe in the death, burial, and resurrection of Jesus Christ” for your faith.
Abraham was justified by faith before circumcision, before Israel as a nation was started, and before the giving of the Law.