Albert Barnes Commentary 1 Chronicles 29:18

"O Jehovah, the God of Abraham, of Isaac, and of Israel, our fathers, keep this for ever in the imagination of the thoughts of the heart of thy people, and prepare their heart unto thee;" — 1 Chronicles 29:18 (ASV)

Keep this forever ... - that is, “Preserve forever this spirit of liberal and spontaneous giving in the hearts of Your people, and establish their hearts toward You.”