This is an idiom that can be translated as "I love you very much." (See: [[rc://en/ta/man/translate/figs-idiom]])
## my partners in grace ##
"partakers of grace with me" or "share in grace with me"
## God is my witness ##
"God knows" or "God understands"
## in the depths of the love of Christ Jesus ##
The phrase "in the depths of the love" is an idiom that refers to the place inside of us that our emotions come from. This can be translated as "with all the love that Christ Jesus has given me." (See: [[rc://en/ta/man/translate/figs-idiom]])