In the case of salvation God has a strict no-return policy. There is no evidence by way of statement or illustration that God has ever taken back from a believer the gift of salvation once it has been given. His love would keep Him from doing so. Keep in mind, Christ came to seek and to save the lost. Faith is our way of accepting God's gift.
Faith serves as our spiritual hands by which the gift is received at a particular moment in time.
Again, saving faith is not necessarily a sustained attitude of gratefulness for God's gift. It's really so simple. God is not looking for a series of promises. His primary concern at this point is not your ability to follow through. He does not want to hear all the things you intend to do for Him.