How important was the Old Testament to Paul when teaching gentiles about Christ and how to live?

2 Timothy 3:14-16 ESV But as for you, continue in what you have learned and have firmly believed, knowing from whom you learned it (15) and how from childhood you have been acquainted with the sacred writings, which are able to make you wise for salvation through faith in Christ Jesus. (16) All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness,