Certainly by dying for their faith the Apostles showed that they were not impostors or hypocrites. They must really have believed what they taught, otherwise they would not have laid down their lives for it. They were certain of what they taught, as we saw when speaking of St. Thomas.
101 Q. Will the Holy Ghost abide with the Church forever? A. The Holy Ghost will abide with the Church forever, and guide it in the way of holiness and truth.
“Abide” means to stay with us.
102 Q. Which are the chief effects of the redemption? A. The chief effects of the redemption are two: the satisfaction of God’s justice by Christ’s sufferings and death, and the gaining of grace for men.
An effect is that which is caused by something else. If you place a danger signal on a broken railroad track the effect will be preventing the wreck of the train, and the cause will be your placing the signal. Many effects may flow from one cause. In our example, see all the good effects that may follow your placing the signal—the cars are not broken, the passengers are not killed, the rails are not torn out of their places, etc. Thus the redemption had two effects, namely, to satisfy God for the offense offered Him by the sins of men, and to merit grace to be used for our benefit.
103 Q. What do you mean by grace? A. By grace I mean a supernatural gift of God bestowed on us, through the merits of Jesus Christ, for our salvation.
“Supernatural,” that is, above nature. “A gift”; something, therefore, that God does not owe us. He owes us nothing, strictly speaking. Health, talents, and such things are natural gifts, and belong to our nature as men; but grace is something above our nature, given to our soul. God gives it to us on account of the love He has for His Son, Our Lord, who merited it for us by dying for us. “Merits.” A merit is some excellence or goodness which entitles one to honor or reward. Grace is a help we get to do something that will be pleasing to God. When there is anything in our daily works that we cannot do alone, we naturally look for help; for example, to lift some heavy weight is only a natural act, not a supernatural act, and the help we need for it is only natural help. But if we are going to do something above and beyond our nature, and cannot do it alone, we must not look for natural, but for supernatural help; that is, the help must always be like the work to be done. Therefore all spiritual works need spiritual help, and spiritual help is grace.