Why does the gospel not apply to infants?
The gospel does not apply to infants because an infant cannot believe or understand it. It is for those old enough to understand right and wrong, recognize they are sinners needing salvation, and believe that Christ died for them. After belief, God seals them with the Holy Spirit.
Les Feldick explains that this verse often shocks people because it seems to contradict what they were taught from infancy. But he points out that the verse cannot deal with an infant, because an infant cannot believe and cannot understand the gospel. The gospel is only for someone old enough to understand right and wrong, to know they are a sinner and need salvation, and to understand that Christ died for them. After they hear and believe the gospel, God moves in and seals them with the Holy Spirit of promise.