I think I have determined a quick and easy litmus test for spiritual maturity: when the challenge is issued--to prayer, to stewardship, to retreat, to service--the question is this: do we hear the challenge as an invitation or an insult? If the latter, our pride says, "What? You think I don't already pray enough? Give enough? Serve enough? You think I am less of a Christian because I don't go on retreat?"
If the former, our humility says, "How blessed to hear a call to more depth and devotion. How wonderful to be given the opportunity to give. How sweet to spend more time with Jesus, to be the flesh of God's Word among others."
Of course, the spiritually immature will hear even this assessment as an insult.