God was warning Israel through the prophet Isaiah about their idol worship.  During this time, Israel had turned from God and worshipped idols and built altars dedicated to them.  God told them to remember the former things and return to Him.  Today, that same warning is still valid for us.  While we may not worship idols, there are plenty of things that we put before God.  We must cast those aside and put God first in our hearts and our lives.