Natural strength is what we receive from the hand of God as Creator. Spiritual strength is what we receive from God in grace.
Watchman NeeOften the children of God cannot rise up to answer the Lord's call to service simply because, though their physical condition is good, their feelings are low, cold, and reluctant. Or even when their emotions are quite high, passionate, and willing, they find themselves unable to serve the Lord because now the body reacts lazily. The disciples found themselves in precisely that situation in the Garden of Gethsemane: "the spirit indeed is willing, but the flesh is weak"
Watchman NeeI need forgiveness for my sins, but I need also deliverance from the power of sin... I appreciate the blessed fact of God's forgiveness, but I want something more than that: I want deliverance. I need forgiveness for what I have done, but I need also deliverance from what I am.
Watchman Nee