The most important thing you can do is to pray for your children. Sometimes, I find, parents pray faithfully for their children when they are young, but then fail to keep praying as they grow older. The Bible says, "Be alert and always keep on praying for all the saints" (Ephesians 6:18).
What should you pray for your children, now that they are adults? First, pray that God will give them a hunger for Himself. Right now, they apparently have little interest in God and see no need to bother with Him. Ask God to work in their hearts, convicting them of their sin of ignoring God and giving them a desire to know Him and serve Him. You can't persuade them but the Holy Spirit can. The Bible says, "When he comes, he will convict the world of guilt in regard to sin and righteousness and judgment" (John 16:8).
Then ask God to give you wisdom in dealing with your children. If you need to ask their forgiveness for anything in the past, don't hesitate to do it. At the same time, they alone are responsible for their attitude toward God, not you. Let them know you love them also and let them see Christ's love in you.
Above all, don't give up. You planted good seeds in their lives when they were young, and in God's time they'll bear fruit.