The KEY to understanding Jn. 6:37- 40 is quite simple indeed – and is found in the included context of Jn. 6:39 - 40.

Jesus declared in Jn. 6:44 that no man could COME unto Him unless he first be drawn by the Father.

John 6:44 (KJV)
⁴⁴ No man can come to me, except the Father which hath sent me draw him: and I will raise him up at the last day.

Therefore, how do we COME (be drawn) to the Father?

Hebrews 11:6 (KJV)
⁶ But without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him.

To this end, how do we get FAITH?

Romans 10:17 (KJV)
¹⁷ So then faith cometh by hearing, and hearing by the word of God.

Thus, when Jesus declared in Jn. 6:37 that ALL that the Father “IS GIVING” (present tense) to Him will COME to Him – this is indeed true and an absolute ALL.  Furthermore, Jesus will NOT banish nor reject a singular one of them in which the Father is giving Him, because Jn. 6:38 reveals Jesus came to do His Father’s WILL and not His own will. 

However, Jn. 6:37 has nothing whatsoever to do with “irresistible grace” – but is predicated upon the Holy Spirit drawing sinners to God through the hearing of the WORD of God.  Hence, FREE-WILL.

John 6:37 (KJV)
³⁷ All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out.

Meanwhile, Jn. 6:39 Jesus declared that of ALL that the Father has given Him – HE (I) should LOSE (verb/1^st person/aorist/subjunctive/active) none.  Hence, Jesus is the "Good Shepherd" (Jn. 10:14-15) and no man can pluck His Sheep out of His hand. (Jn. 10:28-29)

John 6:39 (KJV)
³⁹ And this is the Father's will which hath sent me, that of all which he hath given me I should lose nothing, but should raise it up again at the last day.

Although, PLEASE take note that Jn. 6:39 does not declare nor insist “unconditional grace” and that the ALL Jn. 6:37 Sheep given by the Father to Jesus will therefore ALL be “unconditionally” saved.  Thus, Jn. 6:39 simply declares that Jesus Himself (I) will not be the literal “causal agent” of the LOSS (lose) by any means of ANY of His Father given Sheep. 

Therefore, it is the Father's perfect WILL that ALL the Sheep that He "gives" to Jesus, that Jesus should not be responsible for the LOSS of ANY of them, but should raise them up in the last day.  However, the Father's perfect will is not always the same WILL of Jesus' FREE-WILL Sheep.

As such, Jn. 6:40 reveals those of the ALL Sheep of Jn. 6:37 & 39 that is actually having eternal life and will be futuristically raised from the dead will ONLY be those who are SEEING (present tense) and BELIEVING (present tense) in Jesus.  It is they who are having eternal life (present tense).  This is an individual CHOICE to BELIEVE.

John 6:40 (KJV)
⁴⁰ And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.