Galatians 6:7-9 Be not deceived; God is not mocked: for whatsoever a man soweth, that shall he also reap.  For he that soweth to his flesh shall of the flesh reap corruption; but he that soweth to the Spirit shall of the Spirit reap life everlasting.  And let us not be weary in well doing: for in due season we shall reap, if we faint not.

Whatever we sow we will reap.  If we sow to the Spirit, God will cause the fruit of the Spirit to spring up.  The way that we sow to the Spirit is by yielding to the Spirit.  It is, after all, the fruit of the Spirit, not our own fruit.

If we sow to the flesh, we reap corruption, death.  We sow to the flesh by listening to the flesh and obeying the flesh.

After some people meet Jesus and gain access to the free gift of the Holy Spirit, they don't know what to do with it.  They gain access to grace, but they don't come into submission to that grace.  They receive the free gift of righteousness, but, for some reason, they never allow righteousness freedom to have an effect on their lives.  Rather than yielding their members to the Holy Spirit, they try to do good works by their own fleshly power.  That is the description of sowing to the flesh.  They try to obey the ten commandments by their own efforts.  That means that they sow to the flesh, and by the flesh they reap corruption.

A person who never has met Jesus has nothing but the flesh to depend on.  It's understandable that an unsaved person would be stuck trying to do good works by fleshly power.  An unsaved person would have to sow to the flesh because that person has rejected Jesus Christ and the work Jesus did on the cross.  What about a person who knows Jesus?  Why would a saved person follow the leading out of his or her own heart rather than following the leading of the Holy Spirit?  Why would anyone who knows Jesus try to do righteousness by self-effort and reject God's righteousness.  Worse yet, a lot of people are ignorant of the Bible and they try to make up their own idea of what righteousness is.  Without a clue, these Christians are no better off than a Secularist when it comes to doing right things right, let alone coming to maturity.

Christians were saved so that they can do right things right leading to ever-increasing stability of mind and action, and to final complete fulfillment.  That is the promise of the Bible, and all of this is possible through Jesus Christ.  There is no hope of fulfillment by any other way.  Why would any Christian try to depend on the flesh.  When you depend on the flesh, you sow to the flesh.

Romans 10:3 Ignorant of God's Free Gift of Righteousness, & Trying to Establish Their Own Righteousness, They Didn't Submit To God's Righteousness.