John 14:12 Verily, verily, I say unto you, He that believeth on me, the works that I do shall he do also; and greater works than these shall he do; because I go unto my Father.
In context, the works that Jesus is referring to are the miracles which he performed by the power of God’s Holy Spirit.
John 14:10 Believest thou not that I am in the Father, and the Father in me? the words that I speak unto you I speak not of myself: but the Father that dwelleth in me, he doeth the works. 11 Believe me that I am in the Father, and the Father in me: or else believe me for the very works’ sake.
At Pentecost the apostles received the Holy Spirit and were able to perform miracles. Some are recorded in the Acts of the Apostles. It is reasonable to believe that many more miracles were done which are not recorded.
Mark 16:20 And they went forth, and preached every where, the Lord working with them, and confirming the word with signs following. Amen.
Acts 19:11 And God wrought special miracles by the hands of Paul: 12 So that from his body were brought unto the sick handkerchiefs or aprons, and the diseases departed from them, and the evil spirits went out of them.
Romans 15:18 For I will not dare to speak of any of those things which Christ hath not wrought by me, to make the Gentiles obedient, by word and deed, 19 Through mighty signs and wonders, by the power of the Spirit of God; so that from Jerusalem, and round about unto Illyricum, I have fully preached the gospel of Christ.
Hebrews 2:4 God also bearing them witness, both with signs and wonders, and with divers miracles, and gifts of the Holy Spirit, according to his own will?
We are told that the apostles were able to speak in different languages, heal the sick, raise the dead and not be harmed by venomous snakes. They were also able to confer Holy Spirit gifts upon others by the laying on of their hands.
It is this ability to use the power of God is what Jesus is referring to.
When the Apostles died the Holy Spirit gifts ceased and we are not able to perform miracles today. Those that attain to eternal life in the kingdom of God will no doubt have the power to perform miracles among the mortal population.
We are not able to grow into the full stature of Christ because we are too easily beset by sin. We cannot live a sinless life. What we attain is by grace (unmerited favour) and it is attained by having the righteousness of Christ imputed to us by faith.
Philippians 3:9 And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith:
Galatians 3:22 But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe.
Romans 4:13 For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith.
Romans 5:21 That as sin hath reigned unto death, even so might grace reign through righteousness unto eternal life by Jesus Christ our Lord.
It is because we cannot attain unto the stature of Christ that eternal life is a gift. We cannot earn it by our own righteousness in the way that Christ did.
Romans 3:10 As it is written, There is none righteous, no, not one:
Romans 6:23 For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.
We only attain unto eternal life because God is willing to forgive our sins if we believe and obey the gospel preached by Jesus and the apostles.
1 Corinthians 15:1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; 2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
I hope you have found this helpful.
May God bless you,