To walk by the Spirit is to walk in Christ's resurrection, through His crucifixion, and by the compound Spirit (Phil. 3:10; 1:19b). Christ's resurrection, Christ's crucifixion, and the compound Spirit are one entity. It is impossible to separate these three. This simply means that the compound Spirit in us is the reality of Christ as the embodiment of the Triune God. Thus, in brief, this compound Spirit is the consummation of the Triune God. In a more simple way, the compound Spirit is just the processed and consummated Triune God. This Spirit can never work in anything without the crucifixion of Christ. Hence, this Spirit always puts us to death (2 Cor. 4:10-12). If we desire to have the first kind of walk, we need to be crucified. Likewise, if we would have the second kind of walk, we need to be crossed out, to be crucified. Crucifixion always ushers us into resurrection, and that resurrection is the Spirit. These three things—the Spirit, Christ's crucifixion, and Christ's resurrection—are always together. To walk by being crucified, to walk in resurrection, and to walk by the Spirit are the same thing. Our walk by the Spirit is a walk by being crucified and a walk in resurrection. These three are one.
Whether we are walking by the Spirit or not is determined by whether we are crucified or not. We know whether we are walking by the Spirit by knowing whether we are crossed out or not. If you are not crossed out, even if you feel that you are full of the Spirit, you should not do or say anything. Without crucifixion there is no filling of the Spirit. Therefore, we must immediately take the cross to receive Christ's crucifixion. When we take Christ's crucifixion, instantly we are in resurrection, and the reality of resurrection is the Spirit.
I can testify that I am learning this kind of lesson every day. Very rarely do I answer people's letters immediately after receiving them. When we receive letters, we should ask, "Who will answer—me or the Lord?"
If we have the assurance that we are crossed out, immediately we are in resurrection, and the reality of that resurrection is the life-giving Spirit. Then, whatever we do, we walk by the Spirit through Christ's crucifixion and in Christ's resurrection. This is the Christian life.