1) Take a classic and well-loved hymn of which people are unaware. (This isn't too hard because your punters are aware of very few classic and well-loved hymns. It helps, though, if it comes from a slightly different tradition from your own.)
2) The hymn should be out of copyright. This means it can be chopped and changed at will; no one (apart from you) will need paying. It will generally be projected onto church screens with no mention of the original author at all.
3) You will be adding a chorus, so it is important that the hymn is not too long. Look through it and cut out the verse or verses that seem least singable today; heavy theological terminology especially needs to be pruned. Don't worry if the verses cut were intrinsic to the logic of the hymn; that is going to be different and less clear now anyway.
4) Feel free to alter other words to suit. You may have to do this again after you have written the tune so as to make them fit. Some changes may be to modernise, others will be random, and still others may be simply because someone goofed the lyrics in a key recording, and that became THE version that went out on YouTube.
5) Write a decent, post-Coldplay tune. It must have a new and rousing chorus.
6) The soul-stirring qualities of the song and especially of the chorus must come mainly from the tune, as the words you are adding to the classic hymn will not really say anything. Always remember, when fitting random evangelical cliché phrases into your new chorus, that main verbs generate meaning, and meaning generates controversy. Some song writers even believe that verbs are actually of the devil.
7) Find a single word title for your "new song". It needs to be snappy, contemporary, and possibly related to building practices of which most users of the song will be intensely ignorant.
Purlin? Nah. Bulwark? Better. Foundation? Better still. In fact, I'll stick with Foundation.
Cornerstone's already been taken.