Right, the desire of religious believers to help others is also self-interest. But the difference is that the expected reward comes from God, not from others. That makes it more resolute, because for the secular person if the cost of the care greatly outweighs the benefit of "common decency", then there is no reason to continue. Whereas, for a religious believer, the benefit of carrying out God's will is immeasurable.
As for your last point - we're all sinners and we're not perfect. The calculation is there, but the individual's faith and/or abilities might be lacking.
As for your last point - we're all sinners and we're not perfect. The calculation is there, but the individual's faith and/or abilities might be lacking.