John 6:28 - Joseph Benson’s Commentary on the Old and New Testaments

Then said they Desiring to appear willing to receive his instructions, as well as his bounties; What shall we do that we may work the works of God? Works pleasing to God, so as to secure his favour, and eternal life. Jesus answered, This is the work of God The work most pleasing to God, and the foundation of all others; that ye believe on him whom he hath sent That you acknowledge him for the Son of God, the Saviour of the world, (John 20:31; 1 John 4:14-15;) that you credit and obey his doctrine; rely, for acceptance with God, on his mediation; apply to him for, and receive, his pardoning and renewing grace. See on John 3:16-19. Thus our Lord calls them to a work they never thought of; the owning him to be the true Messiah; the receiving him as such, and trusting in him for salvation, present and eternal, which was necessary, notwithstanding all their acts of obedience to the law, whether moral or ceremonial.