What is eternal life?

Lutero asks:
Why Can't Catholics Explain This?

Who says we can't?

What is Eternal Life?

Eternal life is the the grace of God bestowed upon those whom He deems have merited the grace by keeping His Commandments:

Matthew 19:17

King James Version (KJV)
¹⁷ And he said unto him, Why callest thou me good? there is none good but one, that is, God: but if thou wilt enter into life, keep the commandments.

Revelation 22:14

King James Version (KJV)
¹⁴ Blessed are they that do his commandments, that they may have right to the tree of life, and may enter in through the gates into the city.

Romans 2:1-7

King James Version (KJV)
1 Therefore thou art inexcusable, O man, whosoever thou art that judgest: for wherein thou judgest another, thou condemnest thyself; for thou that judgest doest the same things.
² But we are sure that the judgment of God is according to truth against them which commit such things.
³ And thinkest thou this, O man, that judgest them which do such things, and doest the same, that thou shalt escape the judgment of God?
⁴ Or despisest thou the riches of his goodness and forbearance and longsuffering; not knowing that the goodness of God leadeth thee to repentance?
⁵ But after thy hardness and impenitent heart treasurest up unto thyself wrath against the day of wrath and revelation of the righteous judgment of God;
⁶ Who will render to every man according to his deeds:
⁷ To them who by patient continuance in well doing seek for glory and honour and immortality, eternal life:

and when does it begin in the life of a believer?

God knows.  Perhaps you've heard St. Joan of Arc's response to that question:

2005 Since it belongs to the supernatural order, grace escapes our experience and cannot be known except by faith. We cannot therefore rely on our feelings or our works to conclude that we are justified and saved. However, according to the Lord's words "Thus you will know them by their fruits"- reflection on God's blessings in our life and in the lives of the saints offers us a guarantee that grace is at work in us and spurs us on to an ever greater faith and an attitude of trustful poverty.

A pleasing illustration of this attitude is found in the reply of St. Joan of Arc to a question posed as a trap by her ecclesiastical judges: "Asked if she knew that she was in God's grace, she replied: 'If I am not, may it please God to put me in it; if I am, may it please God to keep me there.'"

Lutero also said:
Please I don't want a hundred different RC opinions on the matter which I could care less about. I want something official from your religion.

Then you should inquire of the Catechism. If you ask in a public forum, you will get many answers.

Sincerely,

De Maria