When we consider the mercy of God, we must always keep in mind His other attributes. Especially that of His Sovereignty. He rules His entire creation, and to be sovereign God must be all-knowing, all-powerful, and absolutely free. God has acted at all times according to His infinite wisdom and goodness. God sovereignly displays His mercy "according to the good pleasure of His will" (Ephesians 1:5,9).
"For he saith to Moses, I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion." (Romans 9:15)
"Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth." (Romans 9:18)
A.W. Tozer remarks concerning God's mercy:
"Mercy is an attribute of God, an infinite and inexhaustible energy within the divine nature which disposes God to be actively compassionate.
God has always dealt in mercy with mankind and will always deal in justice when His mercy is despised.
Mercy never began to be, but from eternity was; so it will never cease to be. Forever His mercy stands, a boundless, overwhelming immensity of divine pity and compassion.
As judgment is God’s confronting moral inequity, so mercy is the goodness of God confronting human suffering and guilt.
We must believe that God’s mercy is boundless, free, and through Jesus Christ our Lord, available to us now in our present situation."
Paul, in his letter to the Ephesians, asked them to remember the condition they were in prior to hearing the gospel - and to remember the hopelessness and misery of that condition -
"1And you hath he quickened, who were dead in trespasses and sins;
2Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience:
3Among whom also we all had our conversation in times past in the lusts of our flesh, fulfilling the desires of the flesh and of the mind; and were by nature the children of wrath, even as others.
4But God, who is rich in mercy, for his great love wherewith he loved us,
5Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved;)
6And hath raised us up together, and made us sit together in heavenly places in Christ Jesus:
7That in the ages to come he might shew the exceeding riches of his grace in his kindness toward us through Christ Jesus.
8For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:
9Not of works, lest any man should boast. (Ephesians 2:1-9)
"But God, who is rich in mercy" changed all that. Now they have "exceeding abundantly above all that they ask or think" for the purpose that "Unto him be glory in the church by Christ Jesus throughout all ages, world without end. Amen"
