It is conditional on believing in Christ. What it is not conditional on is the performance of works of the law.
They turned away from their justification in Christ and to justification through the law. That is abundantly clear in the Bible.
Paul plainly pointed out how they were keeping "days, and months, and times, and years" Galatians 4:10. And Paul makes it abundantly clear that the Galatians were people who had really gotten saved. Yet they fell away from Christ for justification and turned to justification through the works of the law. That messes ups a lot of once saved always saved doctrine.