If a believer sincerely tries to please God and fails, does God reject him?
No. When a believer, under the Holy Spirit's power, sincerely tries to walk pleasing to God and fails, God does not cast him out. The same grace that saved him keeps him, because salvation is by unmerited favor, not personal worthiness. God restores him and helps him continue.
Les explains that a believer stands blameless before God because of His grace, His unmerited favor. That grace is not license to do as one pleases, but neither does it mean God rejects the believer who fails while sincerely seeking to please Him. When the believer is under the power of the Holy Spirit and is trying his best in the light of Scripture to walk pleasing in God's sight, and he fails, God does not kick him out. Les uses the illustration of a parent watching a little child take first faltering steps. When the child falls, the parent does not get upset or give him a boot; instead, the parent picks him up, gets him on his wobbly feet, and gets him going again. That is what God does.