Everyone has always been saved by faith. In the OT, and in the book of Revelation, people were (and will be) required to demonstrate their faith. In the OT they were required to keep the law. In Revelation it explicitly states that they must "endure to the end". Saul lost the Holy Spirit, and did evil in the sight of the Lord. After David sinned, he prayed to God not to take the Holy Spirit from him.
Throughfaith is right. Christians are sealed with the gift of the Holy Spirit. People in the OT, and people who will be going through the tribulation, were not sealed with the Holy Spirit..
First post, gotta jump in somewhere..