We know that a man is saved by grace and not by the law. But this does not mean that grace is limited to our salvation. On the one hand, the book of Romans tells us that a sinner is saved by grace. On the other hand, the book of Galatians tells us that after a man is saved by grace, he should go on in grace. Romans tells us that a Christian begins by grace, while Galatians tells us that a Christian continues in grace. Galatians 3:3 says, "Having begun by the Spirit, are you now being perfected by the flesh?" Hence, a Christian should not depend on grace just for his beginning; he should depend on grace continually.
When a man is saved, he does not need to do anything by his own strength. All that he has to do is trust in God's grace. As he progresses, he still does not need to do anything by his own strength. Again, all that he has to do is trust in God's grace. This is the characteristic of Isaac. It is continuing in the grace of God. Not only is our beginning a matter of grace, but our continuation is also a matter of grace. From the beginning to the end, it is a matter of receiving. In the New Testament our Isaac is Christ. He is God's only begotten Son. He has become Isaac for us so that we can enjoy God's inheritance in Him.
The Bible shows us that there are two aspects of the inheritance which God has given to us in Christ. On the one hand, we are in Christ. On the other hand, Christ is in us. In other words, there are two aspects of God's joining us to Christ. On the one hand, we are in Christ, and on the other hand, Christ is in us. We cannot confuse the order of these two unions. First we are in Christ, and then Christ is in us. This is why the Lord's Word says, "Abide in Me and I in you...He who abides in Me and I in him..." (John 15:4-5).
Our being in Christ has to do with the facts that are in Christ, while Christ being in us has to do with the life of Christ. In other words, our being in Christ touches Christ's work, while Christ being in us touches Christ's life. When we are in Christ, all the facts that are in Christ become ours. When Christ is in us, all the power that is in Christ becomes ours. When we are in Christ, everything that Christ has accomplished becomes ours. When Christ is in us, everything that Christ can accomplish becomes ours. When we are in Christ, we receive everything that Christ has accomplished. When Christ is in us, we receive everything that Christ is today. When we are in Christ, all the works that Christ accomplished in the past become ours. When Christ is in us, all that Christ is and can do today becomes ours.