Say to a stranger: "God loves you and Christ died for you."
If election is false, and free-willism is true, this statement is true. If election is true and free-willism false, then this is a lie to many strangers. In fact, it is a mockery of God's love because you may be saying it to somone God hates, and you are falsely contradicting God.
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Say to a stranger: "God gave his only begotten Son so that whosoever believes in Him shall not perish but have everlasting life." Or simply say, "If you repent of your sins and believe on Jesus, you will be saved"
If election is false, these statements are true. In fact, the first one is a particular favorite of free-willers. If election is true, these statements are still true. The person you are speaking to may not be one of the "whosoever" who believes because we don't know who is elect and who is not, but the statement is still true. Believe and you will not perish. The second statement is also true whether or not election is true. If you repent of your sins and trust in Jesus, you shall be saved. You don't know if the person will repent or not, but you're still telling him/her the truth.
In fact, I really like the way Spurgeon put it: Sinner, trust in Jesus: and if thou dost perish trusting in Jesus, I will perish with thee. I will make my bed in hell, side by side with thee, sinner, if thou canst perish trusting in Christ, and thou shalt lie there, and taunt me to all eternity for having taught thee falsely, if we perish. But that can never be; those who trust in Jesus shall never perish, neither shall any pluck them out of His hand. Come to Jesus, and He will in no wise cast thee out.
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You witness any way you want. If you want to risk contradicting God, that's your business. I would rather witness and present the Gospel in a way that cannot possibly be wrong.