Galatians 5:17

For the sinful nature has its desire which is opposed to the Spirit, and the (desire of the) Spirit opposes the sinful nature; for these (two, the sinful nature and the Spirit) are in direct opposition to each other, so that you (as believers) do not (always) do whatever (good things) you want to do.

Building up our spirit is like building up muscle to make us strong. The stronger we are, the better able we are to resist the temptations and do what is good. When we are drawn away into worldly pursuits, we spend less time with God. We will be weak Christians, easily swayed and tempted. If the goal is to do better as a Christian, we first must learn what it is that we should be doing as Christians.