God's heart is to reach out to the lost; and to save the lost, we share the gospel.
The gospel is the good news that Jesus Christ died on the cross to atone for our sins. In exchange for our supposed destiny that is hell, God sacrificed His own son. Because of that we now have a chance at eternal life in heaven. Through evangelism, we seek to build churches in order to spread the gospel.
"For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life." John 3:16