Some scripture showing faith in the past grace and future grace are both the basis for purity (obedience).

See what kind of love the Father has given to us (PAST GRACE), that we should be called children of God (CURRENT GRACE); and so we are. The reason why the world does not know us is that it did not know him. [2 ] Beloved, we are God's children now (CURRENT GRACE), and what we will be has not yet appeared; but we know (FAITH) that when he appears we shall be like him (FUTURE GRACE), because we shall see him as he is (FUTURE GRACE). [3 ] And everyone who thus hopes in him purifies himself as he is pure. (OBEDIENCE)
(1 John 3:1-3 ESV)

This is what John Piper teaches in "Living By Faith in Future Grace".

By the way, my pastor even warned me about the book. He knew I liked to read John Piper but did not know I have read that book. People who claim that John Piper is a neo-legalist are doing so out of ignorance.

In Christ,

Kevin