Contrary to popular opinion, Samson did not commit suicide. Samson prayed that God would let him die with the Philistines, but he left the matter completely in God’s hands. Judges 16:28-30 read:
Then Samson prayed to the LORD, “O Sovereign LORD, remember me. O God, please strengthen me just once more, and let me with one blow get revenge on the Philistines for my two eyes.” Then Samson reached toward the two central pillars on which the temple stood. Bracing himself against them, his right hand on the one and his left hand on the other, Samson said, “Let me die with the Philistines!” Then he pushed with all his might, and down came the temple on the rulers and all the people in it. Thus he killed many more when he died than while he lived.
Unlike Samson, suicide victims take the termination of their own physical lives into their own hands. God did honor Samson’s prayer, but God didn’t honor Jonah’s similar prayer (Jonah 4:3) or Elijah’s prayer (1 Ki. 19:4)!
In addition, Psa. 66:18 states:
If I had cherished sin in my heart, the Lord would not have listened.
This means at the very end of Samson’s life, he apparently was forgiven. Otherwise, God would not have heard his prayer.
For many more Biblical answers to eternal security arguments, please consult the 801 page volume entitled, The Believer’s Conditional Security by Dan Corner.