Starting in the Spirit, ending in the flesh.

To operate in the flesh is acting to get love/approval/ acceptance. It is acting out of need. 

To operate in the Spirit is acting to give love because we already have it in Christ. It is acting by faith in the fullness of God's love. 

It is not just what we do but ¹why we do it that matters most.

If we are in Christ we already have God's perfect love i.e. God's love for his children is perfect (complete), non stop and infinite because of what Christ has already done. Nothing we do or don't do will add to, take away or inhibit this love. It is now a matter of believing it is ours. 

To more fully experience this love (subjectively), we must remain (abide/ believe) in the objective reality demonstrated by Christs doing all that was necessary to restore us back to the Father (for a further discussion on abiding see link below). 

To start out operating in the Spirit/Love does not mean we will automatically continue to operate in the Spirit. We must not just start in the Spirit/Love but continue/remain in the Spirit (Gal 3:3). Our activity can start in the Spirit and deteriorate into an act of the flesh (i.e. performance, living under the law; living to get approval/love) if we do not abide in His love.

The reason we are called to abide is our tendency is to not abide i.e. to slide into operating in the flesh. In fact operating without the Spirit (in the flesh) is our default way of doing things (i.e. we are naturally inclined to act without the Spirit moving us. Operating in the Spirit however is supernatural i.e. it is being driven by the infinite love of God secured for us in Christ). 

Without the Spirits (Loves) enabling and empowering, we are naturally inclined to operate in the flesh. It's a constant pull on us until we learn to operate under grace i.e. in/by the Spirit/Love. 

To start and remain (abide) in the Spirit requires a constant attitude of ongoing acceptance of (trust in) and dependence on God's love i.e. Christ said, "without me (out of all Christ/God's love for you) you can do nothing" (of a supernatural, love driven nature) John 15:5.

It is the exact opposite of operating in the flesh or what I like to call "performance based" action. Performance based action is acting to gain God's approval and acceptance. Spirit driven action is out of love for God because we already fully have his approval and acceptance/love in Christ i.e. based on Christ's efforts that gained it for us, not our efforts/action. 

Any activity that creates or strengthens a desire to stay focused inward (seeking to meet our need for approval from God or others) and not outward on blessing others has deteriorated into an activity of the flesh and is no longer actions moved or inspired by the Spirit (Love) - even if it started out as an act of the Spirit.

To operate in the flesh is to seek getting what we need i.e. love, acceptance and approval.

To operate in the Spirit is to give what we already have and others need because we derive what we ultimately need (total love, acceptance and approval) from God.

• For definition of terms such as "walking in the flesh" or "walking in the Spirit" click here. 
• For a further discussions of remaining/abiding in his love click here. 

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¹This is why Christ will say to those that had done many "wonderful" deeds, "I never knew you." 

"On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’" ‭Matt 7:22-23

On the surface deeds can look very much the same but come from two very different places.