"The Christian principles are, admittedly, stricter than the others; but then we think you will get help towards obeying them which you will not get towards obeying the others."

Lewis may well have 1 Corinthians 10:13 in mind here: "No temptation has overtaken you that is not common to man. God is faithful, and he will not let you be tempted beyond your ability, but with the temptation he will also provide the way of escape, that you may be able to endure it."