You who are trying to be justified by the law have been alienated from Christ; you have fallen away from grace.
Even now, many Christians believe that by doing this or not doing that, they can please God enough to get into Heaven. And therefore, many obviously believe the opposite — that by not doing this or by doing that, they can displease God enough to get into Hell.
This attitude, it seems to me, voids the whole of God’s pattern by which we are actually saved. Our salvation is not of ourselves, says Paul, lest any one of us should boast. We are saved by God’s grace, made into new creatures, adopted into God’s own family, called brothers and sisters of Jesus Himself because God the Holy Spirit dwells within us.
If we are able to save ourselves, then Christ died in vain. If His blood was not sufficient to wash away sins, literally nailing all of them to the Cross, then Jesus suffered and died for no purpose whatsoever.
Here Paul warns us that if we attempt to keep the law and fail to uphold every letter of that law, we will alienate ourselves from Christ. We will fall away from grace.