You therefore, my son, be strong in the grace that is in Christ Jesus. -2 Timothy 2:1
Timothy was not St. Paul's son according to the flesh, but because both he and Timothy share the same LORD Jesus Christ Who came in the flesh to save poor sinners, they are related in manner the world overlooks.
In this case the sonship has first of all to do with the Apostle's teaching. It was through the Gospel of salvation, both taught and demonstrated by Paul for Timothy, that Timothy became both a son of Paul, and second of all, a child of God, or son, in Christ Jesus.
There is great endearment here. Such is the case because love abounds where there is forgiveness of sins. That love is no more clearly demonstrated than in the flesh of Christ crucified and risen for the sin of the world.
Here the Apostle enjoins his "son," Timothy, to "be strong in the grace that is in Christ Jesus." This strength is not of the sort one would typically count as the world counts strong. Much meekness and outward weakness attends to abiding in Christ Jesus and His grace.
At yet because the grace of God in Christ Jesus preserves both body and soul for the resurrection at The Last Day, it is a source of strength that exceeds the laws of physics as you have come to experience them. It is a strength no earthly power, no matter how great, can overcome.
This grace is in Christ Jesus because it is He who made atonement for sin and rose again from the dead, and then made way for that same atonement and resurrection to be applied bodily to all who believe and are baptized.
He established His Holy Supper that you may regularly participate in His benefits unto eternal life; His holy absolution so that you may regularly hear with your ears the everlasting mercy that has entered into this fallen creation.
You are made strong in the same grace shared by the Apostle Paul and his "son," Timothy. Since you share in the same Christ Jesus as sons, you share in the same resurrection, which is not afar off as some might think, but as near as the Word made Flesh, who dwells with you bodily and in truth.