Do not be deceived; God is not mocked, for whatever a man sows, that he will also reap. For he who sows to his own flesh will from the flesh reap corruption; but he who sows to the Spirit will from the Spirit reap eternal life.
In this verse, we have encapsulated the entire Catholic teaching concerning the idea of "merit." Many people think that when the Church speaks of "merit" she means "extra good deeds you do to make up for Jesus' inadequate efforts at atonement for sin." But this is to completely misunderstand the Church's teaching.
In reality, everything that we do that contributes to our salvation is the result of the grace of Christ, not a supplement to or a cause of grace. And such "things we do" are exactly what St. Paul means by "sowing to the Spirit".
Such "sowing to the Spirit" (for example, seeking God consistently in prayer, or doing the corporal and spiritual works of mercy, or faithfully approaching the sacraments) is not a thing we do to put God in our debt.
It is a thing we do because God is at work in us already and inspiring us to obey him. When we do obey him (of our own free will) we mysteriously discover that we have gotten "bigger inside" and have a greater capacity for his grace. That is, we "from the Spirit reap eternal life".
And so the process continues throughout our lives, growing in grace, responding to grace, receiving more grace till we are fully conformed to the image of Christ. Today, sow to the Spirit. You will reap eternal life.
~ Jeff Cavins & Mark Shea