Who saves man?
Who is it that saves people from their sins? Is it God alone or someone else? Could a lesser being than God save us from our sin?
Scripture teaches that the Father, Son, and Holy Spirit are all able to save sinful man. When we examine this further, we find that this means all three are God.
The Father saves.
Jude 1:25 (ESV) - to the only God our Savior, through Jesus Christ our Lord, be glory, majesty, dominion and authority, before all time and now and forever. Amen.
The INC would not disagree that the Father is a savior; the Bible many times says that we are saved by the Father.
The Holy Spirit saves.
Titus 3:5 (NIV) - He saved us through the washing of rebirth and renewal by the Holy Spirit.
Romans 8:11b (ESV) - He who raised Christ Jesus from the dead will also give life to your mortal bodies through his Spirit who dwells in you.
The Bible teaches that it is through the Spirit that man is saved. The Holy Spirit has the role of regenerating sinful man and giving him a heart of flesh to love God.
Jesus Christ saves.
Luke 2:11 (ESV) - For unto you is born this day in the city of David a Savior, who is Christ the Lord.
1 Timothy 1:15 (ESV) - The saying is trustworthy and deserving of full acceptance, that Christ Jesus came into the world to save sinners
Nothing could be more clear from the Bible than the fact that Jesus Christ is savior. He died on the cross to save man, and countless times throughout scripture he is called our savior.
Only God can save.
Isaiah 43:11,13 (ESV) - 11 I, I am the LORD, and besides me there is no savior.
...
13 Also henceforth I am he; there is none who can deliver from my hand.
The scripture teaches that only God has the power to ransom a soul - only God saves. How do we reconsile this with the fact that both Jesus Christ and the Holy Spirit can save believers? The only way is to declare that the Father, Spirit, and Son are all YHWH, the true God, the one who saves.
Jesus and the Holy Spirit give growth.
One could argue that others play a role in salvation through sharing the gospel and teaching. However, the Bible says that anyone who takes part in evangelism is nothing in the act of salvation.
1 Corinthians 3:6-7 (ESV) - 6 I planted, Apollos watered, but God gave the growth. 7 So neither he who plants nor he who waters is anything, but only God who gives the growth.
Unlike Paul and Apollos, Jesus and the Holy Spirit do not merely play a small role in salvation. Paul affirms that only God can cause growth and that only God can enact real change. When Jesus and the Holy Spirit save, they are causing true growth, actually performing the internal work of salvation as opposed to ministering externally like Paul and Apollos did.
Conclusion
The "Iglesia Ni Cristo" denies biblical teaching and says the Trinity is an invented doctrine. However, as we can see in the case of salvation, the Father, Son, and Spirit all participate together in this divine work, making them all the one true God.
Psalm 49:7,15 (ESV) - 7 Truly no man can ransom another,
or give to God the price of his life
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15 But God will ransom my soul from the power of Sheol