OUTLINE:

“radiance”
from apaugasma [hapax legomenon] – means a light flashing forth (from), radiation, gleam; ref: His eternal light breaks through all the darkness that keeps someone in spiritual ignorance (bondage), i.e. every resistance exerted by sin.

“the exact imprint”
[also translated “expression”] – from charakter –  English derivative is “character” - means an impression, representation, exact reproduction; a graving-tool; properly, an engraving; (figuratively) an exact impression (likeness) which also reflects inner character.

“nature”
[also translated, “substance”] – from hupostasis – means a support, substance, steadiness, assurance;  Usage: (lit: an underlying), (a) confidence, assurance, (b) a giving substance (or reality) to, or a guaranteeing, (c) substance, reality. – Strong’s Concordance; refers to God’s essence

Heb 1:3a [Amplified]
The Son is the radiance and only expression of the glory of [our awesome] God [reflecting God’s Shekinah glory, the Light-being, the brilliant light of the divine], and the exact representation and perfect imprint of His [Father’s] essence, and upholding and maintaining and propelling all things [the entire physical and spiritual universe] by His powerful word [carrying the universe along to its predetermined goal].

Heb 1:3a [NLT]
The Son radiates God’s own glory and expresses the very character of God, and he sustains everything by the mighty power of his command.

Heb 1:3a [ad hoc expansion]
He is the radiance [light flashing forth (from)] of the glory of God and the exact imprint [an exact impression (likeness) which also reflects inner character] of his nature [God’s essence], and he upholds the universe by the word of his power.

Lk 17:5
The apostles said to the Lord, “Increase our faith!”

Mt 21:22
And whatever you ask in prayer, you will receive, if you have faith."

What would you rather have in this life: more faith or more knowledge???

Eph 3:19
and to know the love of Christ that surpasses knowledge, that you may be filled with all the fullness of God.

The Light of God
When you've seen Jesus, His Precious Person, you've see the heart of God the Father. Jn 14:9, Heb 1:3, 1Jn 4:8,16

Heb 1:3a
He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power.