Not only "joy", but "joy in the Lord" is the true security for the believer. "The same things" are the basic doctrinal truths, which Paul always taught orally in the churches and in writing in his Epistles.
There were preachers who did not teach these things, mostly Jewish teachers, who insisted on keeping the Law of Moses (especially with respect to food, Sabbath and circumcision) as essential to salvation. Such teachers are “dogs" (voracious destroyers), "evil workers" (misleading) and "false circumcision" (Jewish ritual). Strong expressions! Some strong! But Paul did not hide the truth for the sake of politeness!
Mosaic circumcision meant the separation of that nation (the Israelis) to God as His people and the required saving their ceremonial and moral law. But the real separation is of the worshippers in the Holy Spirit, by faith in Jesus Christ and not by works of the flesh (verse 3).
This is the experience of Paul himself, when he was Saul the immaculate Pharisee! A true Israelite (not a proselyte), of the honoured tribe of Benjamin; he kept the Mosaic law as a Pharisee (carefully); he was a zealous persecutor of "the Nazarene sect" (Acts 26:9); really, an excellent character ...
However, all this perfection had become a "loss" when meeting Christ; he came to know Him as Lord, who justified him before God by His death and resurrection. Now, the honours and advantages he enjoyed as keeper of the Mosaic law were worth nothing - he just wanted to "gain Christ."
By faith in Christ, the true Messiah and Saviour, comes justification from God. Paul still longed to know this Saviour better – to know the spiritual power coming from His resurrection, even suffering and dying for love of Him, until he experienced in himself the power of one risen from the dead (completely free of sin), as will be the resurrection of believers.
The reason for our salvation is that we may be perfect in our character, through the perfect union with Christ, Who "is also able to save to the uttermost those who come to God through Him” (Hebrews 7:25). By His death He already saved us from the guilt of sin; now He saves us from the power of sin and then He will save us from the presence of sin.
The believer, like Paul, should ardently long for perfection in the second aspect - the salvation of the power of sin in his daily life; he should strive to overcome temptation (from outside or inside) - and God has given us the necessary armour (Ephesians 6:10-18).
We are "perfect" regarding our position in Christ before God - we are "declared righteous" by God Himself, through our faith in Christ (Romans 5:1-2). Let us then proceed to the target – perfection in practice and in works (verse 14), since this is why we were saved by God.
Let us not be satisfied with the progress achieved so far; the target is yet to be achieved and God will give us the clarification needed for each stage of our spiritual progress.
A) "Follow my example" (verse 17). This is not pride on the part of Paul; he, as an apostle, authorized by the Lord and as a believer already experienced and advanced in the spiritual race, could offer himself as an example, which many were already following.
B) However, there were enemies in the churches - libertines who practiced and taught a relaxed and worldly conduct, a "Christianity" without the cross, a "religion" without a new birth. These had selfish interests, whose target was not aimed beyond the things of the world.
C) The citizenship of the Christian is not of this world, but of the heavens. Our Saviour and Lord came and will come again from there. He has already saved our souls and His coming in the future will transform our bodies so as to be in all like Him (see 1 Corinthians 15:51-53). However, let us run, striving to achieve our goal - spiritual perfection in Christ!
In our race as Christians and subjects of the Kingdom of God, let us always keep in mind the fact of the next Coming of Christ; this Coming is not a mere theory of a few "literalists" but a fact declared by Christ and all the apostles, whose writings appear in the New Testament. It is not for us to calculate a date for this Coming - but we ought to believe that it could happen any day, and at any time. Such is the constant teaching of the Epistles (and the Gospels) to stimulate us to true holiness, striving ever more in our spiritual race.
1 Finally, my brethren, rejoice in the Lord. For me to write the same things to you is not tedious, but for you it is safe.
2 Beware of dogs, beware of evil workers, beware of the mutilation!
3 For we are the circumcision, who worship God in the Spirit, rejoice in Christ Jesus, and have no confidence in the flesh,
4 though I also might have confidence in the flesh. If anyone else thinks he may have confidence in the flesh, I more so:
5 circumcised the eighth day, of the stock of Israel, of the tribe of Benjamin, a Hebrew of the Hebrews; concerning the law, a Pharisee;
6 concerning zeal, persecuting the church; concerning the righteousness which is in the law, blameless.
7 But what things were gain to me, these I have counted loss for Christ.
8 Yet indeed I also count all things loss for the excellence of the knowledge of Christ Jesus my Lord, for whom I have suffered the loss of all things, and count them as rubbish, that I may gain Christ
9 and be found in Him, not having my own righteousness, which is from the law, but that which is through faith in Christ, the righteousness which is from God by faith;
10 that I may know Him and the power of His resurrection, and the fellowship of His sufferings, being conformed to His death,
11 if, by any means, I may attain to the resurrection from the dead.
12 Not that I have already attained, or am already perfected; but I press on, that I may lay hold of that for which Christ Jesus has also laid hold of me.
13 Brethren, I do not count myself to have apprehended; but one thing I do, forgetting those things which are behind and reaching forward to those things which are ahead,
14 I press toward the goal for the prize of the upward call of God in Christ Jesus.
15 Therefore let us, as many as are mature, have this mind; and if in anything you think otherwise, God will reveal even this to you.
16 Nevertheless, to the degree that we have already attained, let us walk by the same rule, let us be of the same mind.
17 Brethren, join in following my example, and note those who so walk, as you have us for a pattern.
18 For many walk, of whom I have told you often, and now tell you even weeping, that they are the enemies of the cross of Christ:
19 whose end is destruction, whose god is their belly, and whose glory is in their shame—who set their mind on earthly things.
20 For our citizenship is in heaven, from which we also eagerly wait for the Savior, the Lord Jesus Christ,
21 who will transform our lowly body that it may be conformed to His glorious body, according to the working by which He is able even to subdue all things to Himself.
Phillippians chapter 3