Hebrews 6:5 (KJV)
6 Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God,
2 Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment.
3 And this will we do, if God permit.
4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
5 And have tasted the good word of God, and the powers of the world to come,

Therefore - Because ye have been so indolent, slow of heart, and have still so many advantages.
Leaving the principles of the doctrine of Christ - Ceasing to continue in the state of babes, who must be fed with milk - with the lowest doctrines of the Gospel, when ye should be capable of understanding the highest.
Let us go on unto perfection - Let us never rest till we are adult Christians - till we are saved from all sin, and are filled with the spirit and power of Christ.