This happened at the inception of the church and God was pouring out the Spirit and much grace upon both Jew and Gentile. God increased the church with great numbers in the beginning to make an impact upon the people concerning Christ. These were new converts and were responding to the word and doctrine and the fellowship they embraced and it is a beautiful picture of the unity and oneness they had in the faith and love of God as a new body of believers and that should never be discounted in any way.
The church at Ephesus had this same unity in the love of God but had a relapse and left their first love and was exhorted to go back and do the first works of that love. The church at Ephesus had been tried and tested and was found wanting leaving their first love. Those believer you mentioned in (Acts 2) were very fervent and feared God because of the wonders ans signs done by the apostles (v.43). They had not been tested in their faith and they enjoyed the new found liberty they had in Christ. There is no greater testimony then a transformed life where grace has been imparted to the heart of the believer.
Let me ask you this. If a friend of yours believed upon the Lord and you could see the transformation in their life, you would be thankful and give the glory to God for His mercy and grace. If that same friend was suddenly overtaken by sin or a critical spirit that lasted for a year and the Lord delivered and restored them according to His grace, would not the glory of God be greater in your eyes then at the first? Do not we grow grace (2Pt 3:18) and are we not changed from glory to glory as by the Spirit of the Lord (2Cor 3:18)? If we see a brother overtaken by any fault, even through a lack of love, shall we not first in our heart yearn for their restoration and shall not the testimony of the grace of God in restoring them be more glorious in the eyes of those who believe not?
Remember that these are the ones that God has called and the eternal purpose for calling them...
1Cor 1:23-31
23 But we preach Christ crucified, unto the Jews a stumblingblock, and unto the Greeks foolishness;
24 But unto them which are called, both Jews and Greeks, Christ the power of God, and the wisdom of God.
25 Because the foolishness of God is wiser than men; and the weakness of God is stronger than men.
26 For ye see your calling, brethren, how that not many wise men after the flesh, not many mighty, not many noble, are called:
27 But God hath chosen the foolish things of the world to confound the wise; and God hath chosen the weak things of the world to confound the things which are mighty;
28 And base things of the world, and things which are despised, hath God chosen, yea, and things which are not, to bring to nought things that are:
29 That no flesh should glory in his presence.
30 But of him are ye in Christ Jesus, who of God is made unto us wisdom, and righteousness, and sanctification, and redemption:
31 That, according as it is written, He that glorieth, let him glory in the Lord.