One reason I wanted to reprint your letter is because many parents reading this may be doing what you once did—not just ignoring God, but failing to give their children any moral or spiritual guidance. Perhaps your letter will cause some of them to stop and think again about God and the place He should have in their lives.
The most important thing you can do is to pray for your children. They are adults now, and you can't influence them the way you once did. But God can do what you can't do; He can give them a desire to know Him, and He can draw them to Himself. The Bible says, "The prayer of a righteous man (or woman) is powerful and effective" (James 5:16).
Then, ask God to help you be an example to your children. They know what you were like—but will they see anything different about you now? Will it simply be that you go to church, or will they also see Christ's love reflected in your lives? The Bible says, "Set an example ... in speech, in life, in love, in faith and in purity" (1 Timothy 4:12).
Finally, be honest with your children. Let them know you wish you had given them more direction when they were young, and encourage them to give their lives to Christ.